the most secret skill in kung fu

[Jimbo]

Strength training is noted in all of the old Chinese MA manuals, not to mention the old Okinawan ones also.
It has become the "dirty little" secret in some arts ONLY because of the BS that was propagated in the past ( when MA were marketed to the lazy masses) that size and strength doesn't matter.

Actually, that's a good point.

Particularly around the late 1800s to early 20th century, in China, an 'ideal man' was often viewed as a frail scholarly type who eschewed any type of physical labor. The alternative was a coarse, unrefined, uneducated man. When certain systems were marketed towards these scholarly types, they were often taught that size and physical strength doesn't matter and that qi/chi power would enable them to achieve superhuman abilities. Now, qi is definitely real, but not to the degree of many of those legendary feats.

Also, among some internal styles marketed to the scholarly types, it was believed that even sweating during training was counterproductive.

Similarly, when Eastern MA (such as judo/jujitsu) were first marketed in the West during the late Victorian era, it was often taught that "even a frail woman or small man can subdue a brute with little effort." In these cases, MA became a series of "tricks" designed for specific situations that someone need only remember. This marketing was directed at the upper-crust of society as a new novelty, along with spiritualism, seances, etc.