y not said: “I ask this because the Buddha on more than one occasion stated that such-and-such a person has ascended to a particular realm ‘and will attain release from there’.
It is true that the Buddha has made such statements regarding SPECIFIC individuals.
When people asked him “what is the gati, what is the birth of so and so who just died?”, the Buddha may have said, he had attained the Sakadagami stage before dying and that he will born in a deva realm and attain Nibbana from there.
That is because the Buddha can see that for that individual.
That does not mean it will happen to any Sakadagami. The general rule is that a Sakadagami will not be born again in the human or lower realms and will be born only once in a deva realm. If he/she does not attain Arahanthood there, he/she will be born in a higher realms(s) and attain Nibbana in the future, without ever coming back to the kama loka.