While the Mac-only editor BBEdit isn’t open source, it is a 30-year-old C++/Objective-C project, and BBEdit 14 added LSP support which is largely exactly what the comment you’re replying to suggested: install a language server somewhere on your path and BBEdit will find it and use it. It’s preconfigured for a couple dozen popular ones, and adding ones it doesn’t know about out of the box is usually just as as easy as filling in a couple fields in a dialog box.

I agree with you in principle, but in practice I’m not convinced this is an insurmountable hurdle for other editors.