And there's the issue with it being treated as a criticism of an entire group, and not as a comment on a single person's behavior. There are obviously exceptions to behavioral norms, and as a result any interaction between humans is going to be uniquely contextual. But presenting the concept as a whole as anti-intellectual (or as is commonly done, as some kind of attack on the ability for an enthusiastic person to explain something they are passionate about) fundamentally mischaracterizes the concept. It is not an automatic label that is applied, it's a description of a common and very complex negative behavior.

To explain something needlessly, pedantically or condescendingly and to someone (usually female) that is already versed or even an authority on the topic are the traits of 'mansplaining'. What is happening in the OP, where someone is condescendingly and needlessly correcting a woman (who can be assumed to be aware of 3rd-grade level science like phase transitions given she is qualified to be an astronaut) on her use of a term (that was already a correct explanation) is the issue that makes it mansplaining.

You can be enthusiastic about a topic and share that knowledge all you want, nobody is saying "no don't explain things to girls" (or whatever, I don't think that's what you're claiming to be clear it's just an example). They're saying "don't be rude to other people while explaining things, and this was a rude way to do that".

Edit: Clarity

I guess the question is who they were even talking to. Where they talking to the astronaut, or anyone reading their message. That would make a difference.

If I say: "When the sun rises..." and someone comes along to enlighten me about astronomy and how the sun doesn't rise, that would be mansplaining and not correcting. If they talk to someone else because my words inspirerd them to think about this, then it wouldn't.