That is for perfect mathematical qbits. Real qbits are always noisy, and you need at least 1500 real qbits to emulate a perfect qbit with reasonable certainty,

That is for perfect mathematical qbits. Real qbits are always noisy, and you need at least 1500 real qbits to emulate a perfect qbit with reasonable certainty, putting your number in the same ballpark as the number of 20M noisy qbits from the article.