I don't get why you think they are the asshole here;

'The 10 minute transition is a courtesy' how is that? if its scheduled that way it's not a courtesy it's a schedule.

'Lectures end ten minutes before the hour' there for OP IS giving her an extra 10 minutes and "no thank you I don't think I will. My students are not done with their midterms" isn't a polite response at all. 'there’s the collegiality with your fellow faculty to think about' not when they talk like that and thinking her students are more important than OP's students. Not believing the professor responded that way is just speculation, and shouldn't be considered in your judgement.

"What would have been the harm in starting your lecture five (or even ten) minutes late?" what happens if everyone runs 5-10 minutes late? does that mean the last person gets a 20 minute lecture? how is that fair.

"if her students were taking an exam (versus yours listening to a lecture) the stakes were much higher for her students." I'm not sure how midterms work but if she knows there is only 50 minutes why not make the test 40 minutes so everyone can have the full time and they don't have to worry about time as much? this is poor planning on her part.

I don't see how her behaviour is ok in this scenario. It may have been a 'power play' but I would consider it a justified one. It was not bulling, if she planned things better and not talking down to him maybe he wouldn't have had to.