"Just the interface" is a big deal.

Reddit is the same backend as the Reddit I was using through a third party app a few months ago, but the user experience is significantly worse for me, because the interface I'm accessing the service through adds friction to how I use the service and steers me towards how I don't use the service. Same with accessing email through a web interface versus Outlook versus Thunderbird versus Alpine versus the iOS Mail app.

Lemmy is how I want to interact with user-generated text and comments. Mastodon's interface is not. I don't care that it happens to be ActivityPub on the backend, because the interface drives how I consume and interact with the content.