He's talking about units of the currency, not total supply. What he is really saying here is that there's 2.1 quadrillion Satoshis... Since a Satoshi is 100 millionth of a bitcoin, each bitcoin (21 million) has 100 million sub units to it. There's no increase in supply, only a change in the decimal place.
Note the question to which he had provided the answer.
His answer relates to
economic expansion first and foremost,
not just to bitcoin's divisibility (currently set to 8 decimal places).