I have a counter to your argument: If they didn't have bones, how could they stand in an erect posture?
Oh BTW I like the "Squid stuffed with rice" pics. Very creative.
The same way your dick gets hard.
But if that's how it worked, that would be completely rigid, as they would have no joints.
And to have joints, one must have bones. And as for the ink, not blood, maybe their blood is ink, and another inkling's blood is anathema to them.