If you look into the wiki https://wiki.openstreetmap.org/wiki/Tag:tourism%3Dtheme_park

"is an area where entertainment is provided by rides, game concessions, etc. "

There homepage
https://sealandgov.org/

The history is quite long but it seems to me that the income do not depent on tourism. More it is Blockchan and technologi there is the main income.

So for me it is not a theme park.