if 'good enough for government work' is good enough then the references are valid but an apx 20deg differential seems to me to fall more on the side of 'the one in which the arms and shoulders move in different planes'.
There are VERY few ams that have a bend over the ball of 'at least 35-45 deg' as Hardy says is required for a '1 plane swing'. If you really look at the amount of bend over the ball really required to insure you do not go over the swing plane with no 'drop' of the hands when the first move is shoulder rotation, it is significantly more than the recommended bend for most. The vast majority of ams will go over the swing plane with their first move if actually following Hardy's advice. However if you do what the single plane pros do which is to drop their hands to a lower plane as the first move in the downswing then you'll be fine - but that is what Hardy calls a 2 plane swing
Given the fact that I've still never seen a pro that has a '1 plane' swing per Hardy's definition (for example he says Hogan is an example) I do not think there are different actions for the '2' types of swings. Some of Hardy's direction may work for one person and some other direction for a different person but as Hogan said fundamentally the downswing needs to be on a shallower plane than the backswing and that requires vertical movement of the hands relative to the plane. Even in the case I posted of the pro with the extreme bend over the ball from a recent tour event, he still had that movement to a shallower plane as his first move in the downswing. Even in the case of most tour pros with very short backswings like Allen Dolye, Doug Sanders and Dana Quigley where the hands never make it to the address plane (ball to shoulders) the first move is still a vertical drop to a shallower plane.