It is correct
Case law has defined what needs to be done to done to implement a permission - "significant works" .... which can be foundations or infrastructure. And then there are no limits.
However, there is a provision in the Act to allow councils to take action against development which is, or appears to be, taking too long. This will allow uncompleted parts of the permission to be given a deadline - so no use for extensions or one-off buildings, but more intended for larger schemes