The reason why Immeasurable Speed is given is because the omnitrix was working before the multiverse was created. The fact that this explosion creates time does not mean that the explosion is faster than the time continuum. This is a simple creation of spacetime, and basically the explosion's creation of all spacetime and the multiverse is just a range achievement.
As i said above, since these feats are not immeasurable speed, this reaction speed shouldn't be immeasurable speed and it should be removed.
update:The fact that the Omnitrix is reacting to this explosion, a big bang that created space-time, should not imply immeasurable speed. It does not matter whether this explosion created a hypertimeline or whether it was high-dimensional, because regardless of these dimensions, it represents a feat of creation and cannot be considered as having occurred faster than time. Additionally, there are other aspects of this scene that do not make sense and contribute to its overall incoherence.