Here is an email where Satoshi Nakamoto explains how PoW solves the Byzantine Generals' problem http://satoshi.nakamotoinstitute.org/emails/cryptography/11/
The proof-of-work chain is a solution to the Byzantine Generals' Problem. I'll
try to rephrase it in that context…
On Wikipedia, however, it's said that this problem is proved to be unsolvable https://en.wikipedia.org/wiki/Two_Generals%27_Problem
The Two Generals Problem was the first computer communication problem to be proved to be unsolvable. An important consequence of this proof is that generalizations like the Byzantine Generals problem are also unsolvable in the face of arbitrary communication failures
What did PoW actually solve and with what assumptions?