To be fair, how FastMail does catch-all delivery like this is standard and easily reproduced st any mail vendor (except Office 365) that supports catch-all, which is most of them. I use a catch-all address with FastMail that is @asubdomainichose.mydomain.org and it is the same subdomain I used with my previous setup before moving to FastMail.

Using a subdomain for catch-all is great because spammers can’t easily discover and flood the subdomain.