He actually wasn't strictly wrong about that "better stock market covers the national debt" thing, he just way oversimplified.
Basically if your national economic growth rate is higher than the interest rate on T-Bills, you can effectively sustain infinite debt because the growth in government revenue (assuming that taxation is adequately capturing that growth, such that economic growth is equal to revenue growth as a percentage), as fast or faster than what it costs to service government spending.
Basically if your national economic growth rate is higher than the interest rate on T-Bills, you can effectively sustain infinite debt because the growth in government revenue (assuming that taxation is adequately capturing that growth, such that economic growth is equal to revenue growth as a percentage), as fast or faster than what it costs to service government spending.