You’re correct that patents generally last 20 years from filing. But patent term can be longer when there were processing delays by the patent office. And yes, you can sue on expired patents for past damages (but you can only go back up to six years from the date you sue).

A patent is granted to something new, here the goal was to show that the patent is void, because it was already existing before the patent was (wrongly) granted. It doesn't matter if it was invented less or more than 20 years ago, or even that the original invention was patented.

I think you're referring to the "Earliest Potential Priority Date" column, some of which are older than that? But, e.g., Patent 6,954,431 with the priority date of Apr 19, 2000 (i.e. more than 20 years ago), if you click through you'll see has an "adjusted expiration" of Apr 11, 2022.

I'm not sure why that is, but looking in the timeline I see that "first worldwide litigation filed" was Apr 13, 2020, i.e. a week shy of 20 years, so I'm guessing they filed while it was still just barely active. And I wonder if that granted a bit more life to the patent, because it now expires two years after that.