Yes, but there are two other reasons before that (projected fiscal deterioration, high debt burden). So it's in the list of reasons but, not at the "very top".

A high and growing debt burden on the US government has been true for decades, that hasn't changed, what seems to have more impact is exactly the erosion of governance. The constant stand-offs holding the budget hostage isn't a thing that happens with other AAA-rated governments, it's pretty unique to the USA and has been happening constantly for the past decade+.

I believe what the other comments mentioning "at the very top" mean is that it's there on the by-line, not buried somewhere else in the report. To me it makes sense to call it "at the very top".