Why is Byzantine Generals Problem with traitors possible?

Question

My question is what makes the Byzantine Generals Problem possible to solve in some cases where traitors exist, for example in the case where there are 4 generals with 1 traitor?

If the generals must either all attack or all retreat at the same time, then surely the traitorous general can simply say "yeah sure we'll attack" and then retreat as the other 3 generals plunge into their deaths.

I suspect the answer has something to do with different variations of the problem, but I was unable to find any other definition for the Byzantine Generals Problem other than "all must either attack or retreat". Almost all of the search results are blockchain related advertisement, and the Wikipedia article doesn't provide a different definition for the goal of the generals.