"Nai" already existed in Old Greek, like "einai", which was at that time, not the only infinitive form ending with "-nai". In Homeric Greek, there were several infinitives with "-nai", inherited from PIE where no infinitive existed : those were substantives derived from verbs, just as if you said, in English, "the being" instead of "to be". In the course of the evolution of the Greek language, infinitives in -nai disappeared little by little. It is improbable that Ancient Greeks would have used an infinitive alone as an answer. They would have rather preferred a conjugated form like "esti" = (it) is.