Whether or not the streamers, and general public know it. The Martingale system is widely utilized whist gambling at the sand casino.
Preface; The strategy ultimately fails in the OSRS context as there are limits on earnings AND bets. It is only with unbound wealth, bets, and time that it could be argued that Martingale becomes a winning strategy. The impossibility of winning over the long run, given a limit of the size of bets or a limit in the size of one's bankroll or line of credit, is proven by the optional stopping theorem.
The Strategy;
The theory is very simple. Every time you loose a bet, you double up. Just like with flipping coins (exactly 50 percent probability) the chance of flipping 100 consecutive heads, means that you will eventually win, and thus make your principle back.
The gambler usually wins a small net reward, thus appearing to have a sound strategy. However, the gambler's expected value does indeed remain zero (or less than zero) because the small probability that he will suffer a catastrophic loss exactly balances with his expected gain.
Application;
Individuals such as Spark Mac, with billions in net worth, can reliably use such a system for reliable (low relative to their total net) gains per hour. They will eventually exact the catastrophic losses.
Fantastic video that explains Martigale perfectly!