'Everyone over 30 knows this' is a prior assumption (it is not necessarily correct; and nothing is said about shame).

The comic strip is saying if above is true, then people still have to learn at some point so on average it would be around 10k people per day.

I think the math is this:

For people born in a given year: 4000000/365/30 = 365 people per day

but you have 30 sets of those people (those born this year, those born last year, those born two years ago, etc.) So 365 * 30 = 10950. 10k is easier to say for viral purposes.

I don't think that's what it's saying at all. It explicitly says we should not be negative towards people for not knowing things.

The math on the xkcd is just wrong tbh. In multiple ways.