That number is preceded by the word “if”, right?

It is not. I don't think it said "if" before, but it certainly doesn't say it now:

> When the marginal cost of each student is probably $20K

Seems reasonable, as that's about what K-12 costs at a public school.

Uni has to be way cheaper than K-12? The teacher density is way lower in uni and TAs are not FTE.

OTOH, you don't get room and board at a public school, and the facilities are much less complex (in terms of labs and other specialized buildings).

Is the marginal cost even well defined?

A lot of the expensive universities operate near capacity, where capacity is limited by housing, zoning, lecture hall space, capacity of the departments, etc. They have costs that look marginal (housing, teaching, food) and costs that look fixed (administration, management, some IT expenses, libraries). But, due to the approximately fixed class sizes, this whole breakdown seems a bit fictional.