You might try re-reading it with some charity - the example's purpose isn't to teach the `Maybe` monad, but to remove the redundant check. To go into what `bind` does would be a diversion from the main topic (parsing vs validating).
FWIW SPJ has called this blog's author a "genius" so... I think they do know how `Maybe` works. https://gitlab.haskell.org/ghc/ghc/-/issues/18044#note_26617...