I think it's strange too, but I can see why they would do it this way. In short, it is so as not to spoil anything for USUM. The rules will probably be amended later.
In that ruleset document, they write their banlists according to Alola dex number. Eligible Pokémon are dex numbers 001-204, 206-288, and 293-299. Some quick looking up reveals that the banned Pokémon are Zygarde (205), Cosmog's whole family (289-292), and Necrozma, Magearna, and Marshadow (300-302). If the rules had been written for a potentially changed USUM dex, we'd easily be able to determine the number of cover legendaries/mythicals in those games, as well as the total number of Pokémon in the "new Alola dex".
As for why USUM is not mentioned at all, I guess it's just cleaner that way. Pretend like they don't exist (as seems to be current TPC policy), until they are able to amend the rules in full and include the games and everything associated with them.