If you do a study on Varroa, you’ll find out how their feet hold onto bees. Just so happens the size of powdered sugar fits those feet - pretty much exactly. This causes them to loose hold of adult bees and fall off. Yes, being powdered sucrose, it also stimulates grooming behavior. As well as being relatively innocuous to the bees. However, if a mite gets a good hold of a bee and establishes itself - ain’t no grooming from any bee going to get it off. You have to do something to interfere with it’s grip first. Mites that aren’t affixed, yes - those may well be tossed overboard.
As others have mentioned, using a ‘sugar dusting’ can be part of an IPM, but by itself it is not a treatment. If you’ve an acute mite problem - you have to do something else to treat. Or let them go and try again with different genetics.
My personal treatment of choice is Oxalic Acid Vaporization (OAV). VERY effective on phoretic mites, easy to do, fast and inexpensive. Again, it’s not a magic bullet. You do have to have a treatment plan as OAV has no affect on mites in capped cells. You need to do a ‘staged’ treatment cycle (ie. once a week for 4 weeks) or treat simply when there is no brood present.
Death to Varroa!