It used to be. Mastodon removed it when they rewrote their entire UI in React in favor of their current messy "copy-paste a url into search" setup.
The old follow button would just open a popup where you entered your home instance + username. It would then redirect you to your instance (not needing a sign in cuz you're already logged in on that) and your instance would do all the magic for following.
The old follow button would just open a popup where you entered your home instance + username. It would then redirect you to your instance (not needing a sign in cuz you're already logged in on that) and your instance would do all the magic for following.