These account for 74% of bachelor's degrees issued 2018-2019.

The assumption is that the remainder follow a roughly similar split - they're unlabeled, so assuming the same valid-fake distribution as the rest of the dataset is the best we can do.

found this web site. that lists majors by political leaning. No idea where the numbers come from, but presumably they have a basis other than an assumption.

Presumably. The proportions aren't listed, though, and they don't map cleanly to the other dataset.

His premise that college graduates are evenly split is wrong too. Pew Research found that college graduates skew democratic. My calculations say 59% of college graduates are Democrats.

That's not right. The 2016 election, in which graduates (docs excluded, since we're looking at bach degree stats, and they're negligible in number regardless) skewed more left-wing than usual, was still split 49-45, with 'some college' splitting 43-53 in the other direction.