When using the official Mastodon app (as suggested in the "guide") "instances" are not a factor at all (unless the user specifically goes out of their way). A new user could have never even heard of the term "instance" and the above steps would work fine.

You don’t just download the app, create an account, and go.

Actually with Mastodon this is literally how it works.

EDIT: I should say this is how it works now, it didn't always used to be this way. The official Mastodon app used to ask the user to pick an instance, but for a number of years now it defaults every new account to mastodon.social unless they opt out. There was a big brouhah about centralization but the Mastodon devs felt it made for easier on boarding.