No it isn’t a URL. But that would indeed be the way they could make it work. If they did that, then…

If you don’t have the app installed it installs it from the web site. If you have it installed then the app takes over instead of the web browser. That is how many apps work (eg Reddit).

And it uses same tech as Signal.

However getting friends to join Simplex is complicated by two annoyances:

(1) It gets confused by an invite URL coming from facebook (it doesn’t know to strip the appended Facebook tracking code - as trivial as it is).

(2) When the invite is via a QR code you must scan it with SimpleX not your native camera app. Invitees just give up.

I have exactly once as did a couple of my friends from the same stranger.