That seems like an editorial slip. It’s supposedly based on a unicorn, which does have legendary actions. I’d bet it was originally meant to be a beefed up unicorn stat block to be level appropriate, and somewhere along the way the legendary actions got lost. I’m not sure how much adding the unicorn’s legendary actions to it would change its CR, but that’s what I would do if I thought the combat needed a bigger challenge.
I checked the CR of the corrupted avatar and it checks in at 8 (6 for defense and 10 for offense), so I am guess that editorial error is with the text not the stat block. It was probably changed at some point.
That being said, it is a rather boring monster. I would be happy to make a version with legendary actions (and maybe a bit more) if you want. Actually, regardless of whether you want it or not I will likely give it a crack later today (after work).
It's also just a story - like a Star Trek holodeck episode. Nothing that happens is real.
Anyway, I ran this last week. I gave the corrupted Avatar magic resistance and legendary resistance, but not legendary actions, because: Compared to a regular unicorn CA hits like a freight train, but is squishy. In one round it turned the monk into a wolf and reduced the healer from maximum hp to zero. If it has the unicorn's usual 1LA kick attack it could utterly demolish anything within 5 ft, but the LAs don't allow it to move. In comparison, the shield and the self heal are trivial, doing next to nothing to improve survivability.
If you want to make the CA a real bstd, I suggest giving it these LAs: Misty Step (1), Hoof attack (2), horn ray (3). The horn ray has the wolf transformation effect, but at 60 ft. range.
How the encounter is structured should also be considered. The party is encouraged to rush directly from the fight with the berserkers, which is actually the toughest fight in the adventure, straight to the pool to try and stop the ritual without any recovery time. Then there is a bait-and-switch. The party expect to fight a bunch of werewolves and a priest of Malar, and my lot used considerable resources against them, when they are not the real threat. My players where WTF! when the unicorn attacked them and the (surviving) werewolves ran away.
Overall, this adventure is miss-labelled as level 8 though, it is level 7 at most.