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.

AGPLv3 is the only way to go. If you want to make open source software, you need a license that protects your project and your users from corporate abuse.

I'm almost sure they steal ideas that people write in teams chat too..

This came out a while ago. The developer used a license that said, "Steal this software, I don't care." Then he was shocked Pikachu when it was stolen.

His problem is the exact reason GPL was created.