Lots of major free software projects are hosted by the project creators to avoid being screwed over by proprietary services.
Examples
If Microsoft were to release a competing link aggregator; then they could persuade GitHub (Microsoft’s subsidiary) into locking or removing the main repository for Lemmy’s source code.
Yup, Microsoft can’t relicense the code, it can merely make it inconvenient to access on their platform. It’s really a non-issue.