https://www.forbes.com/sites/seanhanlon-1/2023/08/03/us-sovereign-credit-rating-downgrade-by-fitch-ratings/?sh=1c179e4a5f77

While J6 did factor into it, the rating is on the governments ability to pay back its debt, aka to not default.

The bigger issue, in this context, that I’ve seen is the republicans playing with the debt ceiling and threatening to not raise it. This directly makes it more likely the US government can’t/won’t pay its debt off.