Not back then it wouldn't be, though, if we take the veracity of the story at face value. The inclusion of perfidy (bad-faith deception, such as dressing up as a civilian or opposing force) as a banned act within the Geneva Conventions was only adopted in 1977, under articles 37 and 39 of Additional Protocol I.

Edit: And even today, Article 39 exempts naval engagements specifically from that provision when it comes to flying the wrong colors with intent to deceive.