That doesn't match the SMTP spec, RFC 5321 says
> If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host.
MX implies a domain can receive email, you don't need it to send email. A setup where company.example sends email from companymailings.example but sets a reply-to for support@company.example is perfectly valid (even if it's stupid and confusing). Plus there's that weird legacy behaviour of mail servers delivering to port 25 to the IP in the A record if MX records are missing.
So shouldn’t this be done explicitly by setting a policy at _dmarc.companymailings.example instead of implicitly by setting at otherwise entirely useless record (of type A? some unused TXT variant?) at companymailings.example?