Hiashi can shut down her tenketsu. But he has to do that 64 palm thing first. But even with that we know that a surge of huge amount of chakra would unblock the tenketsu. And that is exactly what byakugou is, a huge amount of chakra.
Also this is something probably similar to the hyuuga tech. With one tap, Tsunade/ Sakura can completely mess up nerve connections, like what she did to kabuto in part 1. Hiashi has no way of undoing the damage.
And this is all without summoning katsuya. Somehow everyone is using hiashi being able to push back one of those juubi arms as a great feat. Yet are forgetting the amount of destruction tsunade/sakura cause on a regular basis without even trying.
There is really no logical reason why Hiashi would be able to beat Tsunade.