If you read the full article, it'll clarifying the full meaning.

If you lend me $10 worth of gold, then I can pay you back $10 of money if it's backed with gold - the gold standard.

If you lend me $10 worth of gold, and I pay you back with unbacked fiat, the repayment is arbitrary.

Like for like you can back any backed money with any asset.

Without a backed money, you can only repay an asset with an asset, otherwise paying with fiat, the global books don't balance.

When I say gold was never used to back debt, this is the debt I am referring to.

Without that context, your statement is true.