How can sole maintainers work with multi-billion corporations without being taken advantage of?

They can't, thats why GPL is noncommercial. Capitalism is an exploitative system that relies on power imbalance. As soon as MS reached out, he should have made it clear they can't even look at his code for ideas without a contract and payment. He shouldn't have told them anything else without a contract. Papers with legal claims on them are the only language business speaks.