Income tax returns will not show his net worth. So your guess about him not being a billionaire will not show up on his tax returns. It has to be something else.
What it is, I think, is they are just grasping for straws. If they get his tax returns, it does not matter what they say: they will lie to the American people and say it says something which it doesn't, and then their idiot followers will believe it without going to investigate the returns themselves. All the Dems are looking for are fake news headlines.