I don't find giving this kind of information meta-gamey at all. The characters probably know of the reputation of dragons as murder-machines, while the players might think it's just a game and everything is level-appropriate. I don't know how the dragon is presented in the module, but the main error seems to be that it doesn't talk about what they characters would know about dragons.Just TELL the players how hard the dragon is!!!
Call it a "passive Nature check" if you want to dress it up in pretty narrative. Maybe if they roll a decent Nature or Arcana result they get a few specifics, too. If they're bad at Nature make sure the druid NPC spells it out for them.
But definitely make sure they know how hard it is even if you have to tell them point-blank "yeah it's a level 8 monster that will wipe the floor with you guys." This meta-game talk breaks immersion a bit, but trust me, your players will appreciate it. Heck, turn it around and make them figure out how the PCs know this.
A game is a series of interesting decisions. Deciding how to handle Venomfang is way more interesting if the players know how tough he is.
I think it's important in an pen-and-paper RPG to have problems the characters can't solve by pure force. Otherwise, it's much too like a computer game. I even like the part where one character has a backstory that more or less gives him a death wish. It shouldn't result in a TPK, but in an interesting role-playing situation where the other characters tells him where to stuff it with his frontal assault plan.
D&D is mostly about cooperation within the party, but I think it's makes it much more interesting if one PC's agenda differs from anothers in a way that leads up to a confrontation* and some serious decision making. It really makes the game feel more real.
*or some sneaky manipulation from one of the PC's, making the rest of the party follow the plan he wants.