Get it? There is no limit... so long as we don't go past the limit!
We already spend ~20% of federal revenue on debt service. How high would that percentage need to get before you would consider it a problem?
So this is meant to illustrate that the limit on debt is more a function of the concrete circumstances. In this line of argumentation, debt is not limited by a mathematical formula that just takes GDP and similar statistics as input.
If you're paying 100% of revenue on debt service, taking on more debt isn't optional. All other federal outlays must either cease or be funded by new debt. Given current levels of federal spending, that would mean increasing the national debt by ~20% of GDP, per year.
More debt --> higher debt service costs --> even more debt --> even higher debt service costs --> et cetera