this is not to mention that you had to register your previous LLC with your state at some point, and it's not like Comcast's lawyers don't know how to search through LLC registrations.

The original registered owner of the LLC is a foreign corporation with multiple owners. Then it was sold. I don't know if you understand how LLC transfer works, but the former owners of a business asset aren't liable for all future incurred debts - since that would be insane - and you don't have to verify that the purchaser is a legitimate entity.

In the world you imagine exists;

If you sold me an LLC, and I stopped paying the bills, you would be liable.

If you and 11 other people owned a foreign company that owned a domestic subsidiary, and sold that subsidiary, and the new owner defaulted on some bills, you specifically among all the owners would be liable to pay those bills.

That assumption is based on a whole lot of nothing.