I get the conspiracy theory. I don't agree with it. He's extremely partisan. And he's doing it to stick out. Most Cabinet Confidence is at the Secret level. Not even TS. That's basically a police and background check. He's been in the public eye for most of his adult life. Very hard to hide a bunch of nefarious activity. So I don't think he's likely to be very risky. He's simply refusing because it gives him a point of differentiation.
Also, given how Trudeau has played this, I don't think Poilievre is wrong to be concerned that security rules could be used to muzzle the Opposition. I wonder how much of this is being over classified just to hide it and make disclosure difficult.