He did monetization the wrong way around, IMO. Most CI is on Linux, but most developers are on Windows or macOS, so he should've capitalized on the Linux builds being paid while the local developer builds on Windows and macOS being free.
Also, mold was designed as an alternative to gold / lld, therefore it would require to be open-source and free on their main platform: linux.
It's AGPL on Linux now, and they sell commercial licenses for companies that won't touch that license, and they were contemplating earlier making mold only available under a non-free source available license like BSL, so there's no "requirement" as such that it be free and open source, even on Linux.
Do you have any data on this ? Maybe that's industry dependent, but I hardly know any Windows (not even talking about macOS, that's almost nil) developers outside of video games and web dev. 100% of Rust devs I know use Linux, to keep on the subject.