Let's say, if the US defaults, do you really believe the bitcoin price will skyrocket or will it be dumped? Personally I don't think bitcoin will explode if the US defaults and I don't think they will default either even though I don't like the US.
It is a valid warning because America's debt has kept growing and interest payments have grown to over $1 trillion. But I suspect that there is a political reason for Elon Musk's alarm. He has openly supported Donald Trump for the US presidency, this might be a plan to give Democrats a bad name. But the US debt has always been increasing even when Trump was president. Trump has openly acknowledged that Musk will be part of his government if he wins. In fact, Trump called Elon a "cost-cutter" who will save the US Trillions. Elon's ability to reduce Twitter staff from 7,800 to 1500 made Trump give him that title.
I don't also think that America's bankruptcy will be good for the Bitcoin market. Bitcoin price is highly connected to the US economy, bankruptcy will affect the market adversely. I also doubt if the US will go bankrupt.