That's a flag at half mast, not a sail, just sayin
You're militarily correct. Nevertheless, nowadays the distinction is not necessarily observed, like on that magazine, and it seems to be tolerated.If we're going to nit pick, I believe, a flag is only at half mast if it's on a ship or Navy base, otherwise it's at half staff.
I'm sure there's a Marine or Navy vet among us who could easily correct me if I'm wrong.