During WoD, most guilds hadn't finished Highmaul on Mythic difficulty before BRF came out. Blizzard has commented on this and stated that BRF did indeed come out too quickly. They took this feedback to heart and increased the time between raids for Legion, offering ToV as a consolation raid while Emerald Nightmare gets to continue until mid-January. An internal decision has been made by the game creators that this amount of time should be adequate to achieve Ahead of the Curve for Emerald Nightmare in particular. As you point out, this means they do in fact regard it as a "quasi-tier," but not by the classical definition. The only difference in Legion is time. The first tier remains to be the set of raid instances offering the first equipment tier set of the expansion: Nighthold and its predecessors, EM and ToV.
By this point I think it's pretty safe to say there will indeed be 3 tiers, with Argus constituting the final tier of the expansion. The only reason Blizzard hasn't commented on the number of raids recently is because they can't make promises yet on whether or not an additional spontaneous raid instance will be featured in the last patch (to combat content drought experienced during SoO and HFC tiers), much like ToV was for Tier 1 of Legion.