Either he means exploiting bonus programs for new members, which is hardly sustainable and unless there's an endless supply of those. Or it's your regular arbitrage gaming, which is also hardly worth the effort and not free of risk. If it's an arbitrage scheme in an exchange, where the quotes might vary over time, your are gambling and at that you are probably playing the bookie who always has the better latency.
There are various sites such as oddsmonkey that do a lot of the effort of finding price disparities for you, so you can arb with confidence.
Regarding bonus arbing, as long as you don't get banned lots of places perform regular stunts with free bonuses for inactive accounts to try to drag punters back in.
(Disclosure: I work for an exchange)
"arbitrage scheme in an exchange, where the quotes might vary over time" - The nature of arbitrage is that the profit opportunity is available at a single moment.
What I interpret your explanation as is more akin to trading - Buying backs at a time and waiting for odds to fluctuate at an exchange for a sale.
Although there is no (simple) way of doing a pure arb between a bookie/exchange, all the popular exchanges expose existing liquidity in a market and you can quite safely reduce the risk by double checking available liquidity at the exchange in the seconds before placing your back bet.
It's a combination of both. Initial signups is not sustainable, for sure, but there are some regular offers from a lot of bookies that are exploitable not for guaranteed profit, but "EV+" opportunities. This is where my consistent side income comes from.
