Well, from the hearings that were made it's already clear that they see Libra as a threat.
Not sure how to put it in terms of debt relationship, they can just use Libra and convert it into dollars whenever needed, nothing changes here.
The threat that I see and how I understand the congress sees it is that if Facebook launches Libra and without a doubt, it will be promoted on Facebook, it wouldn't make sense not to and let's say 10 million people start using Libra and on average they hold a thousand bucks in their accounts.
Thus the size of the reserve that Libra would hold and invest would be 10 billion, which is not huge, but if we ramp up the numbers of user that they manage to sign up, it can grow into a force that governments might need to take into consideration. Governments might be in the mercy of Libra for it to decide to buy their bonds, etc.
It's much easier to stop them when they are just getting started and not once they are already signing up users by the thousands.