Yes, the top-level domain is still just a domain. I'm not aware of any public Internet services which are reachable from a TLD directly, and it's strongly discouraged by ICANN, but there isn't any technical limitation preventing e.g. someone at Verisign from setting up example@com.

In response to your edit.

Yes, or countries could use their cctld, e.g. email@us or noreply@uk.

Or any tld owner could do the same with theirs, of course.

you could also send mails within your local network, the hostname just has to resolve and have a mail service running