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.
It is vital if you are no longer in your twenties and care about health into old age more than simply results. Lack of stability will cause injury very quickly.
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.