All I can think is that he's implying a first baseman's range might get him to an otherwise unreachable ball, but his weak arm prevents him from throwing the runner out, thus cancelling out the plus play. Though unless it's a play at 2B or home, I have no idea why arm would come into play. A flip to a pitcher or 2B covering 1B doesn't take any arm strength.
And I certainly how no idea how you would prove his arm cost him a plus play, as it would just show up as an infield hit in the boxscore.