That means that prosecutors believe Giuliani has potential criminal liability.
That is less likely, for a number of reasons.
In general, I expect anything Giuliani did in concert with Trump as president would be messy for prosecutors to prove. Possible, but less likely.
Prosecutors look for slam-dunk cases that can get them convictions, and that is more likely here if it doesn't involve something he did with Trump.
He should be very worried about becoming part of Parnas and Fruman case, but it's possible that he has much more to worry about. /end