I did have to manually re-enter the host records.
Testing email... looks like the message is blocked by Gmail - not sent to Spam, but actually blocked. Dunno if that's new or if Google have been being arseholes for a while. Might just be because they hate me. Fixing it to go to a non-Gmail account the message arrives fine, so I guess the MX record is ok.
If necessary I can figure out the SPF/whatever records to add, but I'm getting tired so that can wait until tomorrow.
As it was, I'm not sure what (if any) validation was performed - no secret code matching, no email to say "hey, do you agree to move domain X between registrar Y and Z" - only setting the registrar-specific IPS tag; if someone with the same registrar had put in a transfer request at the same time, how would it have known the right one...
*shrug* Guess I should just be happy it worked.
Maybe next year I'll do a multi-year renewal and hand round the begging bowl then. :)
Though that would also be adding another thing which could go wrong, and I presently have enough already gone wrong things I need to focus on resolving for the time being.