It's not. There's going to be a Storymode (Normal), a Tier 1 Hardmode, a Tier 2 Hardmode, and then Nightmare.
I'll see if I can find where I read that. I think it was a Q&A.
I was under the impression tiers meant what you could complete. Nightmare EV or KP is the last tier 1 op and then SM Denova is the beginning of Tier 2 for Ops, same with LI NM being the beginning of Tier 2 for FPs. That is where the breaking point for new gear and harder content begins as far as I am aware.