Yes, that "downhill speed control" could be referring to just this, down gearing to get more engine compression at higher rpms. My F150 EcoBoost does that as well.
But I'm pretty confident that with the Cummins diesel, Nissan will also be using exhaust braking in Tow/Haul mode to get much more effective engine braking. AFAIK exhaust braking is only possible with a diesel. Ford diesel's have built it into the Tow/Haul mode as well.
Considering Cummins has mentioned the turbo system, specifically for the Titan, has the capability to exhaust brake and this down hill speed control mentioned by Nissan, I would hazard a guess the Titan will be able to hold its speed at close to its maximum loading. That rotary valve or even a VNT turbo should be able to adjust the amount of hold back. I hope someone figures out how much holdback it has.
On the ISX 15L, 485hp, it had over 600hp of hold back which was a combination of C-Brake and Exhaust braking through the VNT. I would guess the ISV is capable of around 100-150hp of holdback.
It sure would make sense though, the rule of thumb is never go down a hill faster than you can climb it. Following that rule, without using brakes gives you a significant safety factor.
Gasoline engines can be built to handle exhaust braking, but it would have to be a dedicated build. Engines with extreme valve overlap or light springs wouldn't do so well with exhaust braking though.
