This suggest the administrations claim of a market catasrosphy is overstated

Reuters:

Government shutdown. Federal default. These looming political threats to the U.S. economy might scare investors to buy more U.S. Treasuries in the coming days as they seek a shelter for their cash.

While a protracted government shutdown, and particularly a default, could harm to the image of Uncle Sam's debt, its safe-haven appeal looks unchallenged in the short term.

Worried about rising chances that federal workers and contractors won't get paid if much of the government shuts down on October 1 amid a political standoff in Washington, investors are expected to go by the conventional crisis playbook - dumping assets perceived to be higher-risk and rushing into those seen as lower risk.
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The government will slash spending before it would default on its debt.  The markets know that and they still see treasuries as a safe investment.  If Obama actually challenges that assumption the debacle would be on him.