Like, say, I create an account that reads "email@example.com" and implement something on my fiatjaf.com server so that I can use it to login using Portier?
If you do not own the domain you shouldn't be able to use the domain like that.
My use of Tumblr as an example rather than simply stating hosted-CMS has probably misconstrued my point. (I use Tumblr with my own domain and forget that it is primarily not used that way.)
Another example here is any domain that uses static hosting. Lots of domains are hosted on GitHub Pages these days as the obvious big example (but you can replace that with any other static host like Netflify or raw S3 even). The webfinger spec uses query strings to differentiate user requests which means that a static hosted site is at best limited to a single user/email account (by ignoring the query string and returning a static JSON document).