Yes, that scene doesn't appear in the episode as aired. (The Alchemist
might be able to tell us if that scene was even shot.) And indeed, this "can openers are called 'mommies'" scene is something from the script that actually made it into James Blish's adaptation of "Miri" in the old Star Trek 1
episode adaptations book.
I take it that's a deleted scene. It's very similar to a scene in the James Blish adaptation that is one of the differences between the adaptation and the episode as aired. As such it explains a lot, not only about the Onlies' situation, but about the mindset in the '60s and how the studio and the network would have not wanted the audience to be exposed to what is really a very downer element of the children being left alone.