And more so than that, the issue is that cephalopods are clearly related by every measure we can find (morphology, DNA) to other mollusks like slugs and clams. And are even (although far more distantly) related to us. So it is a bit hard to imagine how this could be the case if they didn't all have a common ancestor here on Earth.

Or all life on Earth was designed by the same engineer. Which would explain the similarities in components and designs.

This is more or less the same argument.

The only difference is perhaps your mental image of the "engineer" and his methodology.