That doesn't sound right to me either. Sounds like a bunch of BS. 'Light road' bikes might weight all of 10 pounds lighter than your Surly, and Sora is usually found on entry level road bikes which are not light.

FWIW I run Avid Shorty 4 cantis on my CX bike with 9-sp 4500 levers which have the same design as the 3500 9-sp and they work fine.