You are correct. As long as your feet start behind the line, one could jump to within a foot of the net and then hit the serve. If your foot makes contact with the line or beyond it before you make contact with the ball with your racket, then it is a foot fault.
This thing about it being the wrong point of the match to call a foot fault is the craziest argument that I have ever heard. The whole point of the rule is to prevent the server from gaining an unfair advantage, regardless the current state of the match.
It was obvious that she was trying to ace the second serve since she knew she couldn't win the second serve on a return. They should have been calling foot faults on her all along. Sort of like how they allow the big stars to travel in baskeball.