It's a shame they don't have an actual application for a truly offline experience. If they had both, people could have their cake and eat it too.
It says 'local first' doesn't that mean you can run it locally after downloading? That's how I set up pianojacq, just so there aren't going to be a lot of disappointed people that lose their practice logs if I get hit by bus #9.
there are desktop builds https://grid.space/downloads.html that run entirely offline. you can also use the center menu to "install" it as a progressive web app.