I am confused because 0 henchmen are monsters in AD1 are there henchmen in other adventures that are?
It is confusing at first. Henchmen (and villains) can be monsters or barriers. Look for a little box labeled "type" on the upper right of the card. It will tell you whether a henchman or villain is a monster or barrier.
Any rules pertaining to monsters or barriers apply to them. For instance, failing to defeat a henchman that's a barrier doesn't automatically do damage to you based on the difference between what you roll and the check to defeat (like it does for monsters). Any damage a barrier henchman does would be described in its powers.