Opsec is sacred in militaries and for good reason. Nobody should ever know anything more than the absolute minimum required to do their job, anything else is information that they could give up or have tortured out of him. Poe was just a nobody fighter pilot, there was absolutely no reason Holdo should've told him anything, especially when she knew there were traitors among their ranks. IMO, Poe is completely responsible for the attempted mutiny, and really should have faced greater consequences