The Haunted storyline of Smallville season 11 had a Monitor outright proclaim this show's Earth-2 was "Earth designate: 2". Oof, but:
I don't know if any of you read the Arrowverse's "The Flash: Crossover Crisis" tie-in books, but their concept of a co-existing timeline becoming its own Earth adjacent to Earth-1 (Earth-1A; as it's said to be in the book's continuity) existed as a result of Flashpoint adjusting to the paradox of it being undone.
I take that mirror box that 'brought' Clark to "Earth-2" as something like that. It created a new timeline from the point Lionel found it after the meteor shower of '89, onwards until the Monitors found it and wiped it out in 2012. Plus, Kryptonians canonically have experimented with 4th dimension time/space (ex. Smallville's Jor-El is said to created the Phantom Zone of this multiverse).
Nerd lingo, basically. But I imagine Earth-167 was considered "the prime universe" adjacent to its branched universe ("Earth-167A"), aka "Earth-2". Not reflective of the multiverse as a whole.