If there was no mark, you'd still have to keep clear if he gybed (rule 10), and he still would have to give you room to keep clear (rule 15).

Rule 18 applies because both of you are required to leave a mark on the same side and at least one of you is in the zone.

Rule 18.2(b) required you to give mark room, unless the boat owed mark room leaves the zone.

Forcing you out of the zone is only relevant if someone else owed you mark room.

It's a subtle way to convert a low/slow give-way boat ("seamanlike") rounding into a right-of-way ("tactical") rounding, but it's not going to happen that often. You lose so much velocity in the downspeed second gybe and since it's directed at a particular opponent, it's essentially a team / match racing tactic.

Tuck this away in your brain's scenario database so you can anticipate it next time.