That's true. Instructed hours can only be counted toward the PPL if they were gained at a registered training facility.
However, registration is not very difficult and there's nothing stopping an instructor registering as a training facility on his or her own.
I nearly did this once but I don't have my own plane and the registration requires that your training aircraft be stated. To re-register every time I got hold of a different plane would be far too much of a pain.
If an instructor has his/her own plane then this is all quite easy. And if the student has a suitably insured plane to learn in then that's quite easy also - so long as they don't mind that plane being registered at the instructors "training facility" as a training aircraft.