The "Create Account" button does indeed let you create a Mastodon account. Just not on mastodon.social. I can't seem to reconcile your description with what happens when you press that button, though.
The official app has a significantly shittier UX than the webui. Third party apps provide a better experience, but randos who haven't already been on mastodon for years don't know that. This is where I've found a lot of the friction lives.
