I don't see the contradiction... If a man explains something to another person in a condescending and nitpicky way, it is called mansplaining. But it becomes blurry if the man explains it not to one other person, which can be assumed already possesses that knowledge, but a group where some people might find that comment not useless or condescending, were it could be a correction or clarification instead.

Astronaut explains excitedly about her experience of the day, with a joke and some not completely factual information while addressing the general public. The 'water spontaneously boils' is not a scientific description but a way to make people interested in learning more about the science behind it.

Here are two perspectives this could be seen as:

1. Man notices that and addresses the Astronaut, explaining to her something that she already knows, in order to raise his own status, through condescending and nitpicking. -> mansplaining
2. Man notices that, assumes the Astronaut knows, but wants to give more information/clarification to the public about this why that happens. -> not mansplaining

From the wording of that exchange, I would think it rather is addressed to the astronaut, so case 1. But this is open to interpretation.