That line would appear to be to relate to repudiating a debt, as in saying it's not validly owed for some reason. Rejecting the debt.
Defaulting isn't the same thing. Defaulting is an inability or other failure to pay a debt that is owed. Failure to raise the debt ceiling doesn't mean we repudiated the debt, or don't owe it, it's just a failure to pay it (for stupid reasons, but that's beside the point).
So, no, that clause of the Fourteenth Amendment doesn't make defaulting on our debts unconstitutional.
"I love signature blocks on the Internet. I get to put whatever the hell I want in quotes, pick a pretend author, and bang, it's like he really said it." George Washington