I think it could make more sense if it's rephrased to 'all code is a liability'.
Then the equation is simpler. More code = more to maintain = more liability. It says nothing about the quality of the code, just the quantity.
Tech debt is a trade-off between near term and long term ambitions - it is literally impossible to build a successful project without accruing this kind of debt, simply because you cannot perfectly anticipate the future. In that sense, it's not so much debt as it is a maintenance overhead.
Then the equation is simpler. More code = more to maintain = more liability. It says nothing about the quality of the code, just the quantity.
Tech debt is a trade-off between near term and long term ambitions - it is literally impossible to build a successful project without accruing this kind of debt, simply because you cannot perfectly anticipate the future. In that sense, it's not so much debt as it is a maintenance overhead.