It's pretty amazing how useless twitter threads can be for actually containing the relevant information, even if it has actually been tweeted as part of it. He has more specific allegations if you dig around his twitter profile for a bit and you can in principle look most of the details up because the development is all public. TBH sounds like this could become a bit of a mess, if it isn't already.
Author of the tweet is the creator of Babel.js
Whilst that provides context it doesn’t explain the allegations.
Yeah that's a big claim to make, from my understanding his point is the money is misallocted while there are bunch of people putting more effort on the project who doesn't gets paid or getting paid lower then Henry. (Based on commits, pull requests, issues etc.).