Or maybe it's just what HAPPENS in real life when you have a son and a daughter, and it's effects can be seem on a societal level through, for instance, the number of male vs female hard science college applicants for instance. Different genders have different interests on AVERAGE. The fact that nature doesn't conform to a idealized view of an equal world does not make it "wrong".

equality of outcome vs equality of opportunity. you are never going to get the first because psychology and biology you can only provide for the second.

Stereotypes don't come from nowhere. Example: https://www.bates.edu/news/2010/12/21/chimp-doll-play/

Here's an even better resource backed by an extensive body of research: https://bigthink.com/the-present/gender-stereotyped-toys/, which is based on https://link.springer.com/article/10.1007/s10508-021-01989-8

Seems more like it's reinforcing what the kids themselves already choose to watch.

It is reinforcing a stereotype.

But it is also telling his reality. It wouldn't be honest for him to say his daughter watches 3Blue1Brown if she doesn't, would it?