What they want is for the guy to tell them what he allegedly told the media, then presumably want to ask him what evidence he has to support his statement.
You might tell yourself the evidence, or the source of it is secret, but I find that difficult to believe. The Canadian government does not have any external espionage service.
They already know what he told the media, and more because they have been attributed to him. Yes, evidence is the real question here. There are many potential reasons why the evidence must be kept secret, you realize we share information with many allies (Five eyes for example). This is just a pissing contest, pretending otherwise is naive or cheer leading.
There may be no evidence and Trudeau is just bullshyting.
There may be plenty of evidence and Scheer is just posturing.
The real point is Scheer is chicken to find out for sure, otherwise he would look at the evidence offered and come back and say it was bull. He won't however because he doesn't have the guts.