I can use myself as one example of the late Dr. Mel Siff's writings in "Supertraining" about "optimal hypertrophy" compared to "maximal hypertrophy." This is context of just broadly saying that lean mass will improve explosive speed. In some it may, but in others it may be a regression of performance.
As a former nationally qualified bodybuilder, I started cycling in 2004 and at that time my stats at 5'6" was 190 lbs with my legs being my best attribute (leg mass).
Since starting cycling my body composition has made some adaptations (refinements) to suit my interests in cycling. My weight now is at 170 lbs and I have lost some lean mass overall, but as my body adapts and refines with less mass my overall ability to sustain a higher endurance speed and my sprinting speed has improved with this refinement (reduction of leg mass). The leaner and lighter I become the more I improve in cycling overall. So for me and those like me the blanket statement of increase leg mass does not apply, however, there are those who the statement will apply.
Dr. Siff speaks about each individual has an optimal muscularity for their peak performance in their particular event. For some it may be to gain lean mass and for other it may be to lose lean mass or it could be lose overall mass (body fat & muscle) or to gain overall mass. So it is important to consider these things when discussing that a general statement may not be the best advice to apply to all individuals.
This is why training specificity works so well. Because the human body has such an ability to adapt to the stresses placed on it. By the very nature of sprinting on a bike the body generally has the ability to adapt or refine to the stress be it muscular hypertrophy for some or become catabolic and decrease in mass for others. Same with marathon running, swimming, weightlifting and so on. Given enough time, applied appropriate training methods and consistency, an individual may change in physical appearance based on how their genetics responds to those factors, but the outcome of the physical appearance is not always the same across the board.
If any of that makes sense or seems reasonable for the discussion. 🙂