A host can also be a subdomain. I'm using Isso on my personal blog for reference: https://blog.posativ.org/. It works exact the same way as for a top-level domain.
Worth mentioning for simplicity's sake, a "subdomain" is a domain, same for a "top-level domain". The period marks simply add another level of terms to a namespace on its left-hand side, usually in an already existing domain's namespace. A domain can have any number of terms that someone wants; it only needs a reachable nameserver, which ultimately the root-level operator controls. To illustrate this, you used to be able to go to http://to/ with the help of any up-to-date ICANN DNS record keeper, but that record got deleted or something (though @to email would still work, as shown in your terminal with `dig to` vs `dig mx to` or `dig any to`).
Right, I managed to enable it for https://www.stavros.io/ in the end, but I did see some weird crashes that I can't reproduce now. Maybe I had omitted the protocol and it didn't like that?