It does say it's valid, but also that it's obsolete, and while the RFC does define valid but obsolete specs, there is nothing defining domains without a dot as obsolete, and it is in fact defined in the regular spec, not the obsolete section

It says valid but obsolete, which sounds like a contradiction to me.

This is technically valid but considered obsolete. RFC 822 allowed domains without dots, but RFC 2822 made this obsolete.

Do email suffix not indicate a different domain like .org and .com for websites?

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.