The goal is not to try to train as specific as possible (if that was the case, you would just do your sport and keep away from the weight room), you still need get stronger in what you do. If your training movement does not allow this to happen in an objectively measurable way, specificity becomes a useless concept. Aspects like time for force production, muscle groups involved in a movement, length of the kinetic chain etc. are more useful in determining specificity in my opinion. Would the guy do powersnatches and bent over rows he would probably get much faster progress for a much longer period of time than with this "exercise".
I disagree.
Remember, strength endurance (Mountain biking/road cycling) is characterized by a combination of great strength and significant endurance. - V. M. Zatsiorsky -
The goal off the bike would be to train as specific as possible in the weight room with exercises which employ that whole triple extension (hip/knee/ankle) as witnessed on the bike, which the OP's exercise has. Specificity can only take you so far (riding the bike), then it would be time for greater strength stimulation (weight room), although specificity (riding) will always remain king. Case in point, my power output on my road bike kept increasing the stronger I got with the deadlift & hip thrust. This is seen with track cyclists, very strong guys that can put out huge levels of power. The OP would benefit more from his exercise than doing tricep extensions etc.
The only point I agree on is there are probably more optimal exercises. Maybe he just doesn't want to do powersnatches and bent over rows.
I can certainly see how this exercise would help a rider out, trust me. I ride bikes all the time, more road than mountain biking, mainly to get my hip flexors super explosive for sprinting. The big difference I find is upper body soreness after mountain biking.