While this might be the case for many of us, there is definitely a group of people who like risk and uncertainty. If you don't look for it you go for a safe 9 to 5 job without any upside. Your salary is fixed and you won't make any big pay jumps. People in the finance industry work a lot based on bonuses, so their salary is uncertain too. Or people working in sales are paid by commissions. There is also a change your salary per month.
Although both gamblers and employees get uncertain income, at least employees don't have the risk of losing money or the worst possibility is that what they did doesn't get rewarded, so they will only experience a loss in time and energy. Whereas in gambling, apart from an uncertain income, every gambler will definitely face the risk of losing their money.
I think what you quoted refers to bonus or commissions on their work, of course having a stable job is different than a gambler in more ways than one and no, salary men have a certain income while gamblers don't have, your quote is referring to being a stagnant employee or not having any aspiration to be better. The proper word for it must be commissions because even if they do not get a sale, they still have an assured payment.