There is separation, at least here in the US, where basically all Montessori schools are private and can choose who to take, so the extreme lows aren't present. The other aspect is kids' abilities are masked by how fast kids grow and change; I'd guess a 20th percentile 4-year-old will be better at things than a 95th percentile kid 365 days younger, so the mixed ages still helps both, because teaching helps someone learn and the younger hasn't learned the thing yet. Where when you get to teens, the more capable student in, say, grade 9 will be able to learn calculus so much faster than a less capable student in grade 12, either you make the former waste most of the class repeating concepts they already solidly understand, or you end up leaving behind the slow learner. You're hurting one, the other, or both by mixing them. But that's doesn't mean they can't both be in art class, or band, or shop together.

Socialization doesn't have to solely occur in an academic classroom.