I'm an introvert, and there are times when I've been either so deep in thought and my own imagination that it takes me a moment to realize someone's talking to me. I've told DH that it's a good idea to get my attention first before asking me a question or telling me something for that reason. So I can see how being an introvert could be in play, if Roe's son was deep in thought and might not have been saying much or even heard the woman speak at first.
It also reminds me of a friend who is even more of an introvert than I am, and is very, very quiet, especially when around others who are talking. In our group in college we had at least two extroverts which meant C just rarely talked and when she did it was usually in a one on one interaction. But one night when we were in a pub on campus during the week, C said something and another guy gasped and said "Wow, she speaks!!" Then he went on and on about how quiet she normally is. Never once actually responded to what C said.
Well then C just got so annoyed by his response that she just shut down and didn't say anything else for the rest of the time we were there.
So I agree, the docent was rather rude to the OP's son, and it strikes me as shaming too, and I agree that it's meant to be shaming.