It sets the stage that they have a mind over matter level of self control, calorie negative longevity diet, self measure along with wanted to "blow off some steam" with bouldering (a high analytical physical activity).
Boulderers don't have large triceps or deltoids...to the extant that arms are used in bouldering, it is in pulling activities that develop the back, biceps, and forearms, not the deltoids or triceps.
You only get large delts and tris without developing other muscles by doing excessively long static planks.
I don't get that part of the joke. Most VCs are skinny AF or quite chubby, but none of them seem to have any semblance of muscle mass?