I always thought the "squatter" description referred to his spending much of his time at MIT and keeping things at MIT without being employed by MIT. (Stallman quit his MIT job when he started GNU out of a worry that if he kept it then MIT might claim ownership of GNU; the legal ramification of the GPL weren't understood as well back then as they are today.) I don't think his use of the word implies that he had nowhere else to live.
I searched the page you linked for "home", "squatter", "vote" and "register" without finding any text the supports anything in your comment. I hope I don't come across as antagonistic to you; I am sincerely curious whether Stallman was ever homeless (i.e., ever lacked reliable access to private indoor space with a door that could be locked).
A quick search of the web also gives me nothing and I don't plan to spend a lot of time trying to prove this. Perhaps this will suffice as a source to satisfy your curiosity:
ADDED. "Until around 1998, my office at MIT was also my residence. I was even registered to vote from there. Nowadays I have a separate residence in Cambridge not far from MIT." https://stallman.org/rms-lifestyle.html
So it took him seven years to get that money. Meanwhile, he was intermittently homeless and only succeeded in registering to vote while homeless due to his fame for his work. He submitted his voter application and listed himself as a "squatter" at his work address. The registrar of voters refused to accept this and changed their mind when an article about him came out in a national publication and said the same thing.
Meanwhile, I routinely hear that software engineers at FANG companies can make upwards of a quarter million annually in salary and he lives in Boston, not known for its low cost of living.
That should be common knowledge for the HN crowd, so I'm not going to try to back that up with a source.
https://en.m.wikipedia.org/wiki/Richard_Stallman