Hmmm... The first rule I found for what I was describing is 18.4, but there is no reason to assume that the starboard boat had to gybe to sail her proper course. (She did gybe, but that doesn't mean proper course required her to.)

Ah, but rule 17 would force the starboard boat to fall off as soon as they became overlapped, because sailing on a close reach would be above her proper course, and it appears the overlap occurred very soon after the port boat finished her tack.

So rule 17 required starboard to turn at the mark.