Zachary: The term "fresco" refers to paint that has been applied to wet plaster. ████ ██████ █ ██████ █████████ █████████ ███ █████ ████ █ ███████ ███ ███████ ██ ████ ████ ██████████████ █████████ █████ ██ ████ ████ ████ ██ █████ ████████ ████ ████████ ███ ████████ ██████ ████ ████ ██ ████████████ ██ ███ ███████ ███████ ██████████ ██ █████ ██ ███████ ██████████████ ███████ ██████ █████████ ██ ███ ██████████ ████ ████████████ ████████ ████ ██ █████ ██████████ ██████ ███ ████████ ██████ ████ ████ ██ ████████ █████
████████ ███ ██ ███ █████████ ██████ ███ ████████ ██ ██████████████ ███ ██ ███ ███████ ███████ ██ █████ ███ ██████ ████ █████ ███ ███████ ███ ██████
Zachary lays out some background and then a recommendation. A fresco is paint applied to wet plaster, and once it dries it permanently locks in the paint the painter used. In the Sistine Chapel, later painters added work that now covers up Michelangelo's original fresco. From this, Zachary concludes that to return the chapel to the appearance Michelangelo intended, everything except the original fresco work has to be stripped away.
Notice that Zachary treats "Michelangelo's original fresco work" and "the appearance Michelangelo intended" as the same thing. That makes sense only if Michelangelo wanted nothing on the wall beyond the original wet-plaster painting.
Stephen points out that painters of Michelangelo's era commonly went back and added painted details to their own frescos after the plaster had dried. His implicit suggestion is that, if that was common practice, Michelangelo may well have added his own details after the fresco dried, and those additions could be part of the look he intended. Stripping everything down to the bare fresco could then remove some of Michelangelo's own intended work.
Stephen's response to Zachary, if █████ ████ ████████ ████████ █████ ███ ██ ███ ██████████
It is impossible ██ ███████████ ███ █████ ███████ █████████ ████ ██ ██████████████ ███████ ██████ █████████ ████ ███ ████████ ██████ █████
Stephen says nothing about whether the later additions can be told apart from the original fresco. His point is about how common it was to add details after drying, not about how hard it is to identify them now.
Stripping away everything ██████ ██████████████ ████████ ██████ ████ ████ ███ ███████ ██████ █████████ █████ ██ ████████ ██ ███████ ████ ██ ███ ██████████ ████████████ ████████ ████ ██ █████
This is what Stephen's response best supports. If painters of Michelangelo's era commonly added details to their own frescos after the plaster dried, then Michelangelo might have done so too, and those additions would be part of the appearance he intended. Stripping the chapel down to only the original fresh-plaster work would then remove some of his own intended work, so it would be unlikely to restore the look Michelangelo wanted.
The painted details ████ ████████ ██ ██████████████ ███ █████ ██ █████ ███ ██████ ████ ████ ███ ██ ████████ ████ ██ ███ █████████ ██████████ ███████ ███████
Stephen's response does not tell us how important the added details were. He establishes only that adding details after the fresco dried was common, not whether those details were a major part of the design or minor touches.
None of the ████████ ██ ██████████████ ███ ███ ████ █████████ ██ ███ ███████ ██████ █████████ ███ ██ █████████ ██████ ██ ███ ██ ███ ███ ██████
Stephen says nothing about the importance or stature of the painters who added work to the chapel. His response is only about how common the practice of adding details after drying was. Whether those later painters were important artists is a separate matter the response never touches, and "none" is an extreme claim with no support behind it.
Michelangelo was rarely █████████ ████ ███ ██████████ ██ ███ ████████ ██████
Adding details to a fresco after it dried does not imply that the painter was unhappy with the finished work. Stephen presents the practice as a routine part of how painters of the era worked, which could be fully planned rather than driven by dissatisfaction.