"impedes" is rose's rule, not grace's. rose is the one in front who flailed her arm a little. but i don't think short of throwing the 'bow or deliberately slowing down and almost grabbing someone, that they are going to call impeding. she did have her arms out but she's running in lane 1 and entitled to most of the space. no, impeding, to me, is like a blocking foul in hoops or obstruction in soccer. getting in the way for the sake of getting in the way even if it hurts your own race.
you can't impede from behind. grace did make contact from behind to try and shove past. but rose left a gap. and importantly rose wasn't sent sprawling or hurt in her finish. nor was the contact excessive. no one was thrown aside. rubbin's racin'.
your theory of what impeding means, seems to say it means any contact. because it probably slowed her down a tenth. good luck handing out DQ for any contact in any of the races that lose lanes. you'll DQ half the field in a serious meet. plus, in relation to how you conceptualize this, i think WA sees it as you "toss" people who basically lose their minds and do excessive stuff, and otherwise just police that people who get messed over by contact maybe get to progress. the contact wasn't excessive enough for DQ and there's no need to put rose through because she finished on her feet and progressed. which to me is how the rules play out in practice. ie, not like the nitpicky false start rules where they would toss their own grandmother if for breaking the 1/10 second barrier of block pressure even if there is no visual false start on tape.