Poilievre is a lifelong politician who loves playing political games. This is just another one of those games. He is attacking Trudeau for not doing enough to prevent China from interfering with Canadian politics. Poilievre demanded an investigation into the matter, which was granted. Poilievre then immediately started calling the investigation fraudulent because the person in charge of it is biased. He has a history of doing this. The last time was when he demanded an inquiry into Trudeau's use of the Emergencies Act only to immediately claim the inquiry was biased and fraudulent when it found nothing wrong with Trudeau's actions.
In order to see the results of this investigation however, Poilievre needs security clearance, which he refuses to get. I doubt he has a secret he's trying to keep (although if he was denied security clearance that would be incredibly embarrassing). I think it's just another game. He can continue calling the investigation (that he demanded) as fake news and claim to be a victim of the evil Trudeau government.
Conservative articles defending Poilievre say that getting the clearance wouldn't change anything because it's not like Poilievre can share his findings after reading the report. These articles claim that if Poilievre gets cleared to read the report, then he now has to shut up about it, which hurts his campaign. I don't really think "Poilievre should only do things that help him get elected" is the greatest defence but it's the one I keep seeing.