> SMTP falls back to A (and AAAA which have not existed back then) when the FQDN does not have corresponding MX record

I think of it the other way around.

Originally, people would specifically say which computer it would be sent to when sending mail, right?

And then later came MX records.

So in my mind it’s like, ok send mail to this host, except if there are any MX records. If there are, then use those instead.