"chance of doubling" refers to...
In my opinion, discussing the chance of doubling a bankroll before losing it should refer to play with some optimal or reasonably-close-to-optimal betting strategy.
Betting the entire bankroll on one hand is obviously not an optimal betting strategy. It gives you a lower bound for the chance of doubling your bankroll, assuming that doubling your bankroll is your only objective. This is the case where you have $500, need $1000 to pay your rent tomorrow, and amounts under $1000 are completely useless. But people have more rational utility functions, and I'd rather have $1200, say, than $1000. My betting strategy is intended to increase my wealth, not simply to double it at any cost.
I certainly agree that with an advantage and a reasonably sized bankroll, playing BJ, the chance of doubling before losing it all is over 50%. But some very small bankrolls doubtless have under 50% chances of doubling. I don't know where the line is.