Or are they immature themselves.

I always assumed this was a common step in the grooming process, tell them they're super mature for their age so that when they proceed to try to establish an adult relationship it feels to be less shockingly inappropriate to the victim

I work in residential therapy. Abusers always find some means to justify and normalize their abuse so the vulnerable youth just go along with it thinking they are the ones at fault. It's infuriating and heartbreaking how many youth I work with that have to be deprogrammed from this kind of thought while simultaneously taught healthy boundaries for other youth and adults. Nothing claws at your soul more than reading about all the awful shit an abuser did or put a child through only for that child to be crying at night begging for a chance to talk with their abuser again.