That's exactly what's meant by insurance not being able to be sold across state lines.

The issue isn't whether Kaiser can sell a Californian a Californian health plan and a Texan a Texan health plan, Kaiser can already do that. The issue is whether Kaiser is allowed to sell a Californian a Texan health plan.

I agree. And that's exactly the status quo for insurance in the US: every policy is subject to the regulatory requirements of the state in which it's sold. "Allowing" policies to be sold across state lines - which, as others have pointed out, is equivalent to preventing states from regulating policies sold within their borders - would reverse this.