You'd need a histological slide to see the effect. I have seen these slides in zebrafish which have essentially the same equipment, a primitive stomach with an attached gas bladder. We have several cases where a fish gorged on scrubbed biofilms (algae) that then resulted in a mycobacteria infection (myco is in all biofilms period). The pneumatic duct then necrosed and clogged with fibrous tissue eliminating the gas connection to the environment so the fish could not regulate its gas content resulting in gas bladder disease.
Here's a link to illustrate
physostome fish:
http://www.bio.davidson.edu/Courses/anphys/2000/Martin/types.html
This link makes it sound like you can visualize this infection on larger fish by examining the gas bladder for signs of infection.
http://www.aquavet.i12.com/Fish.htm
There is considerable research done on physostome fish, pneumado duct inflamation and infection and their effects on larva fish rearing. I think the damage can be done early on at the farm and or can be enhanced by husbandry of the adults.
Again I think it is the over feeding of floating foods or any foods for that mater not the fact that the food floats. I feed almost exclusively with floating food, and occasionally a fish will be gas but starvation clears the issue right way. Occassionally I'll have a fish in a tank that can't seem to handle the level of feed a tank is recieving so it goes into a lower feed rate tank and it's just fine after that... YMMV
You keep mentioning true SBD. What is your reference for so called true SBD vs the fake SBD you see? Are you saying False SBD is just gas? I totally agree and it goes back to over feeding. Fish with gas usually clear with a day or two of fasting. In my book if fasting doesn't fix it, it is SBD.