That addendum is there purely so Bethesda doesn’t sue their asses to oblivion (heh). I am certain it will be very possible to play the mod on a pirated copy but it’s not like they can just write that on their website.

According to their website:

In order to play Skyblivion you will need to own a legitimate copy of Oblivion GOTY Deluxe (ALL DLC) and Skyrim: Special Edition. Our installer will not install the mod unless it can find a non-cracked copy of the original games.

and

Skyblivion will work the same as most other complete overhaul mods. Simply install the mod via the installer and press play.

So you’ll probably need a pure unmodded version of Skyrim for it to install onto, but we won’t know for sure until the actual release date.