I got a temporary fix in place for mine. Here is everything I did in case anyone needs this urgently.
Followed steps A & B from link above as zimbra.
Made a copy of smtpd_sender_restrictions.cf and saved it outside of Zimbra folder as a backup.
I added this line to bottom of /opt/zimbra/conf/zmconfigd/smtpd_sender_restrictions.cf
Made a copy of main.cf and saved it outside of Zimbra folder as a backup.
I added these lines to bottom of /opt/zimbra/postfix/conf/main.cf
After doing zmcontrol stop & start it was working for me.
smtpd_restriction_classes = restrict_tpmail
restrict_tpmail = check_recipient_access hash:/opt/zimbra/conf/tpmail_recipients, reject
The necessary line was added to smtpd_sender_restrictions and the two lines at bottom of main.cf were still there.
This definitely cannot be the right way to go about it and I didn't expect it to even work since main.cf is supposed to be rewritten.
Looks like it's not always fully rewriting the main.cf file and only rewrites the lines it is told to. I'm guessing smtpd_sender_restrictions comes from that other file.
I wanted this working by morning so until I hear of a better way it'll work.