Why would Apple be supposed to do this? Without any other argument on the table, taking that statement to its logical conclusion, it would imply Apple is supposed to help make every single project run well on their hardware.

Even when restricting that to “open source projects that (cl)aim to run on latest MacOS”, I think that’s not something a vendor is “supposed to do”.

So, if Apple only “is supposed to” support some projects, what objective criteria should it use to pick them?

I think it’s completely fine if Apple wouldn’t support any open source. I also think it’s completely fine if developers would choose not to write software for their OS because of that.