Just to play devil's advocate, it could also mean that Flynn is the one guy they've got nailed dead to rights. The burden of proof to take down Trump himself is going to be so high, they might not want to grant immunity to the guiltiest guy in the whole bunch, and risk not having anything stick.

Sure, if you're so confident in your case against Trump you don't need Flynn's testimony, maybe you don't give him immunity. But that's a ballsy decision when you're gunning for the sitting president, even with an ironclad case. IMO, the Occam's razor explanation is: you aren't sure at all you can bring down Trump (or even how knowledgeable the idiot was about the most incriminating details), but you know Michael Flynn is a devious fucking traitor who belongs in prison, and your case can put him there. In that situation, you aren't looking to offer immunity either.

To be clear: I hope to God I'm wrong. But I've seen no evidence that any Republican official is willing to do more than wag a finger at him, and I've seen this sub become an echo chamber of misguided optimism about Trump's demise, and about FBI investigations, before.