It's also significant that they can prove he's lying, which would indicate a body of evidence that is probative to Mueller's exacting requirements without his testimony. That's a significant departure from the usual standard of conspiracy cases in which people flipping is about the only way to move the investigation up through the hierarchy.
Alternatively, they have discrepancies between his story and another guy who flipped, and they're using it to punish him with harder sentencing. Sounds like they can verify though.
That would be one specific form that body of evidence could take, yeah. There just usually aren't so many witnesses to specific acts of conspiracy that you can do this sort of comparison with this confidence at this stage, which would suggest that, provided there's something there, it would not have been covered up with sufficient competence to hide, say, Trump's involvement.
In other words, there's usually a certain amount of circumspection and obfuscation here. Trump may well have just said, in the middle of a campaign meeting, something like "Hey Poptartopolis, loop Paul here in on the treason thing we're definitely doing with Wikileaks. I want to be able to tweet about it tomorrow."