Yes wtiger, you are exactly right.
The other theories about how powdered sugar might work are simply wrong.
If you go back through back issues of ABJ, you will find the original article
on this in June of 2000.
Mites have "tarsal pads" - little suction cups.
Sugar particles in the 5 micron to 15 micron range will clog those pads,
and the bees cannot hold on to anything (bees, comb, whatever).
This is why it is important to create a fine "cloud" or "mist" of powdered
sugar when "poofing" it on the bees. Anything less is just dumping
sugar in the hive. There have been several attempts to make the
treatment less labor intensive, and while they may have some level
of success, they have not been subjected to "controlled studies" to
prove that they are effective approaches.
Here's a complete run-down I wrote back in 2001:http://listserv.albany.edu:8080/cgi-bin/wa?A2=ind0105D&L=BEE-L&P=R1576&I=-3