What happened before reaching the zone (the lee-bow) would only matter if rule 17 came into play (which it didn't) What is more important is that the boats were both on starboard tack, overlapped at the zone of a mark that was to be passed to port.

Rule 18 does apply, as none of the exceptions in the preamble or rule 18.1 are met. Therefore, the outside boat (O) is required to give the inside boat (I) mark-room.

If you look at the definition of mark-room, however, you'll see that it says "...mark-room for a boat does not include room to tack unless she is overlapped inside of and to windward of the boat required to give mark-room...". Because boat I was overlapped to leeward of boat O rather than to windward of her, boat O is not required to give boat I room to tack.

Boat I could have gone head-to-wind in an attempt to shoot the mark, or to encourage boat O to tack, and boat O would need to keep clear. Boat O is not under any requirement, however, to let boat I tack.

From the description given, it sounds like boat I sailed into a bad position (albeit by a windshift) and boat O closed the door on her cleanly.

I hope that helps,
Eric