It is allowed, it just doesn’t happen instantly, as you point out yourself.
There are no “board specific moderators” (forget all about what you know from “traditional” forum software ), and (global) moderators do not necessarily subscribe in e-mail form to all messages.
I don’t receive e-mails—is there a way to report forum spam via the e-mail interface?
This is in the spirit of keeping the discussion tidy:
For saying “thank you” and nothing else, you can simply use the ️ icon button, as that FAQ says and @InvisibleMan78 pointed out.
Discourse is more than just software, it’s also a way of/philosophy for handling discussions, and the software is very much built around these ideas. This leads to things such as posts requiring meaningful content to be allowed (e.g., more than X characters, no "thank you"s), keeping deleted messages around for review (as well as keeping changelogs of edited posts).
The Discourse team wrote a blog going into more depth about their “Universal Rules of Civilized Discourse”, which is an interesting read if you’re into this kind of thing:
The MetaBrainz forums rules haven’t been altered from the default Discourse ones, as they are basically 100% in line with our global Code of Conduct anyway.