Execution of Leg Sets

[Scott Stevenson]

It could be, but I haven't seen his material addressing the squat specifically, rather the leg press where he mentions most people go too deep and shift tension onto low back etc via rounding at the bottom.

That's an entirely different issue related to performing the exercise "improperly" meaning in a way that predisposes the low back to injury, not an issue with the normal recruitment patterns used to execute the lift.

Have you been lumping squat, leg press and hack squat (machine) together in terms of quad activation over the course of the movement (ROM)?...

-S