The terminology in the text misses the mark a bit.

- P and NP are not disjoint sets. NP just means "verifiable in polynomial time." There are plenty of easy problems to solve that are verifiable in polynomial time. P is in fact a subset of NP.

- NP-hard problems are ones that are difficult for computers to solve

- NP-complete is the intersection between NP-hard and NP

Furthermore, it presents the P vs NP problem as an attempt by mathematicians to solve P = NP, whereas almost no mathematicians believe P = NP, they're just all trying to prove that P != NP.