30 is assuming you write code for all 30 days. In practice, it's closer to 20, so 75 tests per day. It's doable on some days for sure (if we include parameterized tests), but I don't strictly write code everyday either.
Still, I agree with them that you generally want to write a lot of tests, but volume is less important than quality and thoroughness. The author using the volume alone as a meaningful metric is nonsense.