That makes absolutely no sense as an architecture decision.

It makes a lot of sense architecturally. It's much simpler to update a webpage than force every user to update their app when something needs to change. Consequently a lot of apps, especially on mobile, use a system webview with a page loaded from a server and rendered to look like the app for anything that gets regular updates. The alternative is having lots of versions of the page and needing to manage all the different variations of payloads from them.

Backwards compatibility in an API isn't that hard, but when there's a simpler option (a webview) people will take it.

That said, if there's no native stuff in the app, it is a bit harder to justify.