Is it necessary to be Buddhist to reach enlightenment?

Is it plausible or possible in Buddhist thought for a being to experience nirvana, enlightenment, without adhering to Buddhist thoughts or viewpoints? It it possible for people to receive true dharma from a not-Buddhist framework? From my standpoint, there are many people in the world from various traditions who practice deep meditation and live virtuous lives. It seems reasonable to me that in their practices they, too, come into the presence of the deepest grounds of being, and achieve profound realizations and transformations. Yet, if they are not Buddhist, they may not report back things like emptiness of self and all phenomenon, or reincarnation. They might instead discuss the soul, the self, God, for example. Hence my questions. In some Mahamudra texts, I have read things like “one must recognize the emptiness of the mind” in order to get to the most advanced levels. Is that really so? What happens if we get to the deepest levels in meditation, and we decide that there is not emptiness of the mind? I would imagine the answers are different for different schools of Buddhism. I am interested in specific schools’ thoughts, and general Buddhist perspective as a whole.