Okay let’s break this down a little bit (or a lot) 
First of all… single floating point precision starts to become significantly imprecise at about 7 digits so that means the “usable” space in UE4 is up to about -+ xyz 1000000 cm (1048527 is the default map size limit) so unless in Ark they are using a custom coordinate system to support larger maps in multiplayer then I doubt it can be much larger than that, at least it didn’t feel like it to me when I played the game.
1000000 cm = 10 km but since we have positive and negative coordinates that means the map size is effectively 20km across.
Now here comes the part that I think gets confusing :
The terms “square kilometer” and “squared kilometer” are technically not the same thing! even though they are commonly used as if they are (imo wrongly so but then English is not my first language).
A square kilometer (symbol km[SUP]2[/SUP]) is a unit of measurement for surface area so in this case we would say that the UE4 map size is 20km[SUP]2[/SUP].
Now the square of number (as you hopefully know) is the result of a number multiplied by itself, to “reverse” the process you find the square root.
So when we say 20 squared it usually means that it’s the square root of the number (so √20)… NOT 20[SUP]2[/SUP] which would be 400 (20[SUP]2[/SUP] = 20 * 20 = 400).
So when they are saying 36 km squared they probably mean 6 km across which sounds just about right to me
EDIT:
A better explanation might be written like this:
(20 km)[SUP]2[/SUP] = 400 (km[SUP]2[/SUP])
That can be read as: 20 kilometers squared = 400 square kilometers.
But the pronunciation is not exactly standard which is probably why you wanna use symbols when it comes to math as much as possible to avoid such confusions
So…
36 (km[SUP]2[/SUP]) = (6 km)[SUP]2[/SUP]