"2pp" only exists by tradition. Properly speaking, the first party is WotC, the second party is the customer buying the game, and everybody else are third parties. A "2nd party publication" would be a letter you write to WotC complaining about their lunatic practices.
We call Kalamar and etc. 2pp because they feel semi-official, so the gaming community calls them 2pp as something between 3pp and 1pp even though it makes no gramatical sense to do so.
People elevating WotC publications above officially liscensed publications above everyone else's publications is not any sort of official rule, just tradition. We made that tradition to create a shorthand for what is allowed in a particular game. i.e. so we can say "all WotC and all 2pp material allowed, and only these two 3pp books that I like allowed, stop asking". Since the reasoning for elevating WotC above other publishers is that WotC is thought to have better quality material, that reasoning would still apply, so all pp will remain the same pp as before.