Yeah, the whole "designed for" qualifier can get confusing sometimes. I wondered the same thing when I saw this tonight.
I guess it's like the LAR15 pistol magazine holds 10 .223/5.56 rounds but just happens to fit into an AR style rifle.
So I wonder if there was some sort of magwell conversion kit for this to make it Glock compatible, would it be permissible to use the G17 mags on it? Or is it because the firearm is classified as a rifle and not a pistol caliber carbine that it can legally only use pinned-to-five mags?