I agree the grip is certainly part of it...I don't agree the flex is 0 to do with it, especially as lifters get stronger, otherwise Lifters who pull with straps (Strongmen, mostly) wouldn't seem to be pulling more on the Deadlift Bar than a normal one...but perhaps at 900lbs plus we wouldn't be disagreeing about the flex anyways?
And, by the way, I certainly don't like people saying "Using X adds Y pounds to the bar" - like it's THE LAW - under almost any circumstances.
Even in the unhitched ones, strongman sometimes uses even longer bars with larger plates typically stacked out to the end of the bar. This isn't always the case, since some of the deadlift ladder events are on normal DL bars using calibrated kilo plates.
As you mentioned, the benefit of the bar based solely on starting height of the pull ranges quite a bit. If you have a lifter using super sumo (something you can only do on a 7.5' bar), and they grab it near the middle, they will be getting a bigger advantage than someone like me that grabs the bar on the power rings and pulls conventional. Sumo is typically weakest off the floor, and most don't "rip" the bar off the floor. They slowly squeeze it up, which again probably offers some kind of advantage. For the guys who grip and rip, probably not so much.
It is also worth keeping it in context: that number is in regards to your total in a competitive sense, in relation to a weight class. As he says, steroids is probably going to mean at least one weight class jump, so your relative performance is at least somewhat curtailed. In terms of absolute gains? He has made clear it is much greater.
I think I get maybe 20-25 lbs out from a deadlift bar compared to a decent quality regular gym bar. The main advantage to me is an easier grip, as the bar is thinner with a more aggressive knurling.