According to a 23-reputation user, they are (now) unable to comment on an answer to their question. They are only presented with the "Follow-up discussion" and "Say Thanks" options. I can see the user previously commented on the question, prior to the experiment going live.
I don't have a sock puppet account with <50 reputation and a question (with an answer) to test if what they state is true.
Upon further discussion with the user, they stated:
It didn't allow me to continue with Request Clarification as I don't have enough reputation!
So this looks like a bug, and the experiment is now blocking users with <50 reputation from commenting on answers to their questions.