Well, Lemmy’s front end talks to it’s back end via an API, which is pretty much how every web app of any scale works these days.

There’s really not any particular difference between a monolithic app vs a seperate front end in terms of “all those API calls” since everything is basically calling APIs at this point, if they’re not made by complete incompetents. (In the case of Mastodon, though, I suppose complete incompetent is possible.)