You are somewhat correct but not 100%
First off Serato DJ DOES NOT support DVS without a DVS supported interface. This includes DJM-900SRT but not the DDJ-SX.
Furthermore the DJM-900SRT can work with Traktor just like anything can................ most likely. I mean you can even get Serato ONLY controllers to work with DVS (Traktor Scratch) if you have the know how. One of Traktor's downfall is one of their strong points in that they let you use pretty much ANY device as well as ANY sound card but this same thing is what leads to many of its stability issues.
I have the 900SRT coming it this Tues. being able to use it as Traktor Sound card that is. My guess is it will operate the exact same way as Nexus........................ I'd be willing to bet this i fact. It's just too similar. The SRT ONLY ADDS without taking away anything!!