Isn’t webview-based still essentially chromium-based?

Is Apple Music the same as iTunes?

This can happen to any store unfortunately. If a publisher withdraws, you’d no longer be able to download it. Qobuz has this too. Some publishers are quite scummy with this and upload a slightly different version, which no longer qualifies as the album you’ve purchased. You need to download it asap after buying.

It is, but I found it strange not seeing it in the actual purchaseable music section

Qobuz also does purchaseable music, not just streaming.

They don’t need Signal to do any of this though, so this doesn’t seem like a very plausible theory.