Not arbitrage in the literal sense. The article claims only a small edge over thousands of equal risk positions which means no trade, or pair of trades, is risk free.
I'll add that very few things, even those that are arbitrages in the theoretical sense, are truly risk-free.
The classic example being the arbitrage between on-the-run/off-the-run treasury spreads. Bonds in the current series ("on-the-run") tend to trade at a slight premium to equivalent bonds that were issued earlier ("off-the-run"). Theoretically this is a perfect arbitrage. Short sell the expensive bonds and use the cash to buy the cheap bonds. Sit back and collect the spread.
However, you have to understand why the OTR spread exists in the first place. Investors value the higher liquidity that comes with the current series. Consequently during liquidity crunches, the spread will blow out to multiple times higher losses. In these scenarios arbitrageurs face deep mark-to-market losses, margin calls and investors withdrawals. This very strategy was (one of) the "pure arbitrages" that blew up LTCM in 1998. The market can stay irrational longer than you can stay solvent.
So, even what looks like a simple mechanical science in practice is an art requiring experience and judgement. You have to know the right leverage to use and at what times. You have to keep your finger on the pulse of the market and have a sense of when spreads are too tight or loose given macro conditions. You have to secure good funding relationships. You have to be smart about keeping powder dry so you can buy at cheap prices during dislocations. And so on.
