That jives with my sense that META is a mediocre company
There's nothing wrong with well-founded and thoughtful criticism. On the other hand, it is very easy for this to turn into personal attacks or bullying - even if it wasn't intended to be.
If you're not careful you'll end up with juniors copying the style and phrasing of less-carefully-worded messages of their tech demigod, and you end up with a huge hostile workplace behaviour cesspit.
It's the same reason why Linus Torvalds took a break to reflect on his communication style: no matter how strongly you feel about a topic, you can't let your emotions end up harming the community.
So yes, I can totally see poorly-worded critiques leading to HR complaints. Having to think twice about the impact of the things you write is an essential part of being at a high level in a company, you simply can't afford to be careless anymore.
It's of course impossible to conclude that this is what happened in this specific case without further details, but it definitely wouldn't be the first time something like this happened with a tech legend.