When a couple of Poles spent an afternoon at Treblinka and reported that they found no mass graves, they meant that they didn't find any mass graves filled with bodies. That is consistent with the story that the Germans dug up all the bodies, burned them to a crisp, reburied them in the empty pits, dismantled the camp, plowed the surface, and planted lupins to erase any trace of the camp.
Evidently after the Germans left, the local peasant farmers descended on the site to search for the gold and other valuables that everybody (except the Germans) knows can be found at any site where lots of Jews are killed. In their frenzy to extract all the gold they used the numerous unexploded artillery shells laying around the Polish countryside to blow the site to smithereens.
This, of course, resulted in the ash and sand and bones and chunks of putrefying flesh scattered over two hectares that the Polish investigators said they saw. It would have also made it impossible to distinguish any specific mass graves which they wouldn't have been able to find anyway because the Germans hide them.
They didn't find any mass graves per se but they did a visual inspection and claim to have seen but didn't actually document an unspecific number of things that might be consistent with dead fauna in the area. That's how we know 700,000 Jews (or maybe it was two million) were murdered there.
It's a minor technicality but "didn't find any mass graves" doesn't mean they didn't find any mass graves. It means they didn't find any evidence of mass graves. And, as one Einstein here said, "absence of evidence isn't evidence of absence."
