I'll extend some comments I made downthread:
a16z is a major investor (growth round resulting in a board seat for a16z) in many of the companies that have extended vesting periods, such as Coinbase, Pinterest, Asana, and Tilt (list here: https://github.com/holman/extended-exercise-windows). They also invested in CodeCombat and I'm sure some other startups on that list (I didn't check them all). So I seriously doubt they block companies from doing extended exercise windows.
And claiming that those companies were/are hot and therefore had leverage is a bit circular because hot companies are the kinds of companies a16z often funds in growth rounds (and often it is because a16z funded you that makes you hot).