Posit: the term "magazine clip" is a SEO hyper-optimization, because it allows people to find what they're looking for whether they call it a clip or a magazine.
There are some other possibilities:
1. They have no idea what they're talking about, and "magazine clip" sounds better than "clip clip" or "magazine magazine". I wouldn't put a gun company above doing something stupid. There's the infamous scan of an H&K USP ad showing rounds loaded into a magazine backwards.
2. They're confused, and are thinking of a dual or triple magazine holder/band that allows magazines to be held side by side. Although I've never seen one of those things called a "magazine clip", I suppose that might be a logical term for it.