I think you meant multiverse at the end, and that each timeline is its own universe, but in any case, I don't agree anyhow...
We have the Primeverse and PrimeSpock and Nero time travelling into the past to "create" (or simply track-jump) into the Kelvinverse.
We have the MU, which has an unclear divergence point from the Primeverse, and MUSpock and Nero apparently time travelling into the past to create/track-jump into the MUKelvinverse.
As the divergence point of the MU predates the divergence point of the Kelvinverse, I don't see a reasonable way in which the Kelvin MU could be identical to the Prime MU...unless the MU was itself created due to time-travel...
You are right about the term multiverse but there would not be a mirror Spock to go back and create a mirror Kelvin and very little chance Nero is there to follow whoever goes back bit most likely no one does and they just let Romulus explode.
But there would be nothing to stop Kelvin Kirk from visiting TOS mirror anyway and still have Kelvin mirror stories but Kelvin mirror would not be Kelvin mirror until Kelvin Kirk touched it