Dumbledore disagrees. The type of timetravel that happens in HP: Prisoner of Azkaban is self-consistent. Same as with Rick’s timetravel in the snake episode of Rick & Morty (specifically excluding the snake time travel, which is an example of non-consistency, leading to endless paradoxes).

Now ofc having a timeturner and/or a timemachine would be the impossible part there, but again, CTC’s are technically allowed by GR.