You mean doing it myself? I don’t have experience in this unfortunately, but aside from that, the readme says that it can’t be built without some keys that they won’t provide. I’m also unsure if it would count as a trademark violation to publish it as is without their consent.
Maybe a starting point would be to email them and see what kind of response we get.

Oh I see, my bad, I just remembered wrongly from back when I used both repositories alongside each other

Interesting, I wonder if the 2 facts are somehow related, I don’t know what their stance was on the free software side of things, though I may have some emails saved from GitHub notifications that say something, I’ll look for those.

Edit: I don’t have any on the topic of F-droid inclusion, unfortunately, just one about language support

We are a small team and are not looking for people who can contribute to the project

This bogus argument is always big no no for me, clearly if they’d just been a small team they would not only accept, but enjoy whatever kind of contribution they can get from external people.
They just don’t want to deal with the community and do whatever they want, I’m guessing.

they don’t want our help, their loss

Spot on!

Glad to know I wasn’t alone on this, it is a shame really

Good because there’s enthusiasm for being able to use YouTube music without paying the subscription, but I agree that it’s a shame everyone seems to be more into doing everything by themselves rather than contributing to a single piece of software and keeping it stable, featureful, but most importantly working.
Up to now I probably switched apps 6 times: Vimusic -> Innertune -> Innertune fork -> SpMp -> Rimusic -> Harmony Music (Plus others that I just tried for a few days at most)