Let's keep this simple. We're talking about tanks, so you can take the term 'non-buffer' as implicit.

Let's say that we have two tank players with identical dps(t) curves prior to raid buffs. We already agree that they both have the same rDPS regardless of team composition, and the proof of this is trivial from the formula itself. Let's say that I come along with Arcane Circle and I apply it with the same timing to each of these curves. I get the same personal benefit from each of these players being on my team, which is the quantity that I'm actually interested in as a buff provider. In fact, you can do this with any buff, be it single target or raidwide, and always get the same result. That's really what we're talking about with dps parity on tanks.

Their aDPS could be anything, because that depends on the party's composition. And the aDPS benefit of having a BRD present doesn't even correlate with two minute burst.