I don't agree.
IF the student has shown to be able to do multiple PFLs with good success (like normal, with the throttle closed/mixture rich) and IF you have a nice wide, long and quiet runway available, I can certainly imagine that the instructor would pull the mixture to ICO and let the student perform a proper glide landing. For instance as the capstone landing after a training session on PFLs.
Provided that the instructor watches the student like a hawk and doesn't let something silly happen, like dropping full flaps too early, this can be done safely, IMHO.
And it's a great confidence booster. We all know that with the throttle closed the engine still delivers a bit of power, and if things go bad, you can always go around. But to pull off a successful PFL with the mixture closed, you really have only one shot. Plus, the flight characteristics of the aircraft are for real - there is no idle thrust. In a sense it can have the same confidence booster value as sending someone first solo.
And hey, gliders do this all the time, safely. You just have to know proper heights, glide angles and so forth.
But I agree that PFL training by default should be done with the throttle closed. Using the mixture to stop the engine should be an exception, and only under the conditions described above.