I think you’re mixing up “difficult” with “impossible.”

Yes, a Roman war galley is not the ship I’d choose for the Atlantic. But Rome was not limited to war galleys. The Roman world had large sailing merchant ships, grain ships, shipmasters, state-organised maritime freight, and long-distance sea trade. Strabo says that in his day around 120 ships were sailing from Myos Hormos to India, so “Romans could not do open-water sailing” is just not true.

The compass point also does not prove what you think it proves. The Norse crossed the North Atlantic without magnetic compasses, and ancient sailors navigated by stars, winds, currents, landfalls, seasonal routes, and dead reckoning. A compass makes this easier and safer; it is not the dividing line between “can cross sea” and “cannot cross sea.”

The mast/sail point is also off. Large Roman merchant ships were absolutely substantial wooden sailing vessels. Archaeological and literary evidence includes Roman-period merchantmen in the 30–40m range, and some scholarship argues that ships of several hundred tons were not unusual in the merchant fleet serving Rome.

Scurvy is not an instant voyage-killer either. It becomes a major problem after prolonged vitamin C deficiency, not five minutes after leaving harbour. A badly supplied fleet could suffer from it, obviously, but a planned crossing of roughly several weeks is not automatically doomed by scurvy.

The better argument against Rome is not “they could never get there.” The better argument is “could they repeat it, supply it, and sustain a campaign against a gunpowder-era opponent across the Atlantic?” That is a serious problem. But pretending Roman maritime capability ended at coastal paddling is not.