It's called a clawback (1) and as a policy are pretty common. I wouldn't assume we have the full story from GP, it could be the startup screwed someone over, it could be the person did things that triggered a clawback. Not all are enforceable.
The problem is that even if a company illegally triggers a clawback, it's a civil matter and requires the employee to pony up for legal action against the company. That might be worth it if you're already certain that your equity has significant value, but most people aren't going to litigate for relatively small amounts of stock in small startups with uncertain futures.
(1) https://www.mystockoptions.com/content/how-does-a-clawback-w...