It sounds silly, but I'm not enough of a paper nerd to know if bamboo is normally in our paper.
If ballots were replaced, you might be able to spot differences. But that assumes only one source of paper was used to create the original ballots, and I don't know that that is tracked to that degree.
Like anything else though, it is just suspicion until you get proof. But they aren't looking for actual fraud at this point. They are looking to continue to fuel the doubts. They need this to fade out slowly so they can stoke the fires come next election. They don't want it to end suddenly and for the less indoctrinated people to realize it was all BS.