This is a funny thing to say to your daughter. Especially the daughter that you’ve just put in the position of pretending devotion. Unless he’s not saying it to Ophelia, but to Claudius. Claudius answers him and certainly takes it to heart.
My text has a note that says (To Ophelia) before “read on this book” and doesn’t switch back and add a (To King) before this one. The king’s a much more likely candidate but a Polonius who says this to his daughter is a very particular sort of Polonius.
Also? Sugaring over the devil calls to mind many delightful images featuring the little red horned cliché devil. In one, he’s covered in powdered sugar, like a Devil Powdered Donut. In another, he’s getting a sugar rub massage and he’s got that massage sugar all over him. In another, he’s a caramelized demon. Or he’s covered in honey. Or he’s got that sugar wax on him, about to have all his hair removed with those waxing strips. Or frozen in an ice tray popsicle cube. Sugared and frozen.