Have you seen the pictures of the garrison that did surrender? I suggest you examine them. Truk was doomed whether Japan surrendered in September or not.
Again, the point isn't that Truk was doomed. It's that the soldiers did not surrender until instructed to, by the emperor, despite their doom being obvious to anyone with a pulse. Truk could be bypassed, so it mattered little whether or not the garrison surrendered. The home islands couldn't.