I don't think this is true. I've been following a fairly standard progression on several of the standard exercises over the last year and half. I've seen steady progression on leg press, which is a strongly stabilized and isolated exercise. I saw the same rate of initial progression on squats but then it dropped off and I haven't really seen any progression for six months.

The issue is stability. I have to provide the stability for squats. The machine gives me stability for leg press. I won't get the stability I need for further progression, at least not at an optimal rate, just from squatting. I need to do complementary exercises.