The Telegraph article cited the difficulties being in secondary schools, not primary, and also mentioned yr 9 in particular. Presumably that's parents who thought they could afford it but don't feel secure enough now, and so have decided now to move so there is no disruption to GCSEs.
The demographic drop has not reached that year group.
And indeed has not reached secondary age at all, though it will in the next year or two, hence plans to reduce capacity in a year or two, which will probably be able to go ahead unaltered in places which do not have high numbers in - and therefore possibly leaving - the private sector.