Well, CI is where the costs are, and if the application is big enough, even a few percent reduction via faster linking times would equate to lower costs, while in contrast, developers won't really care or notice a few percent reduction on their local machine.

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.