> I found it interesting that not even this article thinks about it that the US mainland ever can be attacked

It can be, but it would be very, very difficult for anything short of lobbing ICBMs around. You'd have to have a fleet of ships that would be detected as soon as they set sail, and then protect that fleet for the entire voyage, which would also be extremely difficult for any adversary.

Getting boots on US soil would be even tougher.

Drones are an option, but cross-ocean ones are not an easy problem to solve.