It can’t be nibbana itself, because in AN10.7 Sariputta specifically says that this perception/immersion has arisen. And nibbana is not something that has arisen.
There are other problems with this interpretation too, such as perception being limited to the six senses and dependent on contact (phassa), which nibbana is not. (SN22.56)
Such problems are avoided when Sariputta’s perception is understood to be is a cognitive reflection on the cessation of existence, rather than being equated to nibbana itself. There are other indications for this too, such as it being called a “reflection” (paṭisañcikkhati) alongside other reflections such as the reflection that the body being liable to sickness. This too is not a direct experience but an indirect understanding of that fact.
The way Sariputta’s perceptions are phrased also indicates they are cognitive reflections. E.g. “nibbana is the cessation of existence” is an understanding of the type A = B, which is a cognitive understanding, not a direct experience.
The closest direct encounter of the cessation of existence while alive, is the cessation of perception and feeling. There are many places in the suttas that effectively equate this state to nibbana. There is nothing beyond this state, the suttas also indicate.