In the presentation he showed in game "out of the box" clocks of 2050mhz with a reference cooler, and then immediately after said there will be "15, 18, 20% oc headroom"
It's not an AMD "overclocker's dream" situation, which some people apparently took to mean "zero overclocking headroom but it looks like it could overclock". Nvidia provided specific numbers.
Unless they are straight up lying, the 1080ti is going to hit 2300-2400mhz.
Edit: I may have misinterpreted what he meant by "out of the box". The pictures do say "2ghz oc", but I took his demonstration of 2050mhz clocks in that ue4 demo to be literally "out of the box", which I guess it wasn't.