I like the way the artist not only chose a crack pairing but also one they don't seem to care about at all. This is peak yuri-as-commodity.
I'm kind of boggling at the idea of these two being a crack pairing when I've read a fair amount of fanfic with them. Songwriter-composer is a pretty natural combination and there's lots of ways you could justify them making a connection.
That said, you're absolutely right about this basically being a commodity. You would literally take this story and plug any character into it with exactly the same dialogue. Neither of them are particularly in character anyway, so Maki x Umi works in it the same as, I dunno, Erena x any of the stage manager girls. It doesn't matter, they're both written as generic as can be anyway so why not?