Adarq.org

    It does, however, imply that a trainee should not move beyond more fundamental, hypertrophy-based training if they have no intention of competing / testing any time soon.

I largely agree with that. I do think there's value in mixing in some heavier training just to maintain/improve skill with heavier loads, but most of your training should include enough volume to provide a solid hypertrophy stimulus.
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They were trained males, not specifically powerlifters. I'd wager that curls are EASILY one of the most-performed exercises among resistance-trained males.

    They would likely have poorer bicep MU recruitment with an isolated isometric than they would with a compound

Nah. MU recruitment is essentially maximal in single-joint lifts. You can see that in studies that assess voluntary activation. Subject produce as much force as possible isometrically, and then electrodes zap the motor nerve (while the subject is still exerting maximal force) to ensure that every MU is firing. If the zap increases force output quite a bit, that suggests that the subject wasn't achieving maximal MU recruitment. In studies on young folks without neurological or serious orthopedic conditions, voluntary activation is essentially 100%

Just for some more background information here (specifically for the people concerned about the use of curls, or people concerned about the MU measurements being performed at submaximal intensities):

First, neural adaptations aren't synonymous with technique or general motor skill. In a research context, neural adaptations are generally adaptations that affect the contractile force of a muscle per unit of input from the nervous system. For example, if you recruit more motor units at a lower relative force output, if you can achieve higher motor unit discharge rates, or if a given relative increase in motor unit discharge rates results in a greater relative increase in force output, those are all indications that neural adaptations have occurred. Technique and general motor skill, on the other hand, more-or-less refer to your ability to get everything you can out of your muscles in a more technically demanding exercise.

The current dominant thinking is that trained lifters have better technique/general motor skill than untrained lifters, and that neural adaptations have occurred, such that they achieve greater motor unit recruitment and higher discharge rates (and thus greater force output) per unit of input from the nervous system to the prime movers. However, those assumptions about neural adaptations are largely based on studies that simply assess surface EMG amplitudes, which may not actually reflect "true" neural adaptations (https://www.researchgate.net/publication/262019289_The_increase_in_surface_EMG_could_be_a_misleading_measure_of_neural_adaptation_during_the_early_gains_in_strength).

For assessing neural adaptations, movements that require minimal motor skill and technique are preferable, because you're attempting to separate performance due to neural adaptations from performance due to motor skill and technique. And, specifically, isometric contractions are preferable because they have an even smaller skill component than relatively simple dynamic contractions, and because they're necessary for actually decomposing an EMG signal using HDsEMG, in order to study the behavior of individual motor units. With dynamic movements, your muscle fibers move a bit relative to the electrodes on the skin, so you can't isolate the signals of individual motor units. Furthermore, submaximal contractions are preferable, because they let you assess motor unit recruitment thresholds over a larger range of target force outputs, and changes in motor unit discharge rates in a larger pool of motor units.

So, this study found that:

1) motor unit recruitment thresholds are the same between trained and untrained people at all intensities assessed (pushing back against the idea that trained lifters can recruit a larger proportion of MUs at submaximal forces)

2) motor unit discharge rates are the same between trained and untrained people at all intensities assessed (pushing back against the idea that trained lifters perform better by achieving higher motor unit discharge rates)

3) the relative increase in force output per unit of increase in motor unit discharge rate is the same in trained and untrained people (pushing back against the idea that trained lifters perform better by achieving larger relative increases in force output per unit of neural input)

4) the strength difference between trained and untrained individuals in a low-skill task (maximal isometric elbow flexion) was reflective of differences in elbow flexor size: ~65% difference in force output, versus ~70% difference in anatomical cross-sectional area. This suggests that there's not any neural magic occurring somewhere between the highest submaximal intensity tested (70% of maximal force) and maximal intensity; if there was, the trained lifters should have achieved greater maximal force output per unit of muscle size.

All of that strongly suggests that resistance training doesn't improve your ability to recruit motor units, achieve higher motor unit discharge rates, or squeeze more force out of your muscles per unit of motor drive in a general sense. So, when people chalk improvements in performance up to "neural adaptations," they're probably just observing improvements in general technique and motor skill with the specific tasks being assessed. The adaptations required to coordinate a complex movement (i.e. general technique and motor skill) are not the same adaptations required to increase force output per unit of neural input. I do suspect that, say, hamstrings MU recruitment increases as someone gets more experience with deadlifts, but that's reflective of improvements in technique and motor skill, not the nervous system's inherent ability to recruit MUs in the hamstrings or achieve high MU discharge rates.