Indeed, ~2200 umol/m²/s is cloudless sun directly overhead. Your typical blazing tropical dry season sun, your typical desert summer sun. Any clouds, even wispy ones, and any angles, reduce this.
That said, if these studies (there's not just one, multiple come up with the same number) are correct, then it's not just an issue that citrus gets stressed by full sun - it's an issue that citrus can't use the extra energy at all, regardless of how much stress there is on the plant. The sun can only add stress.