The teaching of original nature (benxing 本性) is not the same as the teaching of nirvana. First, nirvana is one of the core teachings of Buddhism while original nature is itself a Chinese term we can connect to the teaching of tathagatagarbha, and tathagatagarbha is something we find only in specific Mahayana sutras. Understandably, non-Mahayana schools never even had the concept of original nature and even in Mahayana it was something only a few traditions took up. And here is the second reason, that original nature doesn't necessarily fit into the teachings of a tradition. For instance, both Madhyamaka and Yogacara - the two main systems of Mahayana - has teachings contradicting the idea of an original nature. Or, even when in some cases teachers of those schools accepted tathagatagarbha, they came up with their own interpretation of it. It was in Chinese Buddhism that because of these reasons they actually said that besides Madhyamaka and Yogacara there is a third major Mahayana thought, Tathagatagarbhavada, i.e. those who take original nature as a fundamental teaching.
"There is no such thing as the real mind. Ridding yourself of delusion: that's the real mind."
(Sheng-yen: Getting the Buddha Mind, p 73)
"Neither cultivation nor seated meditation — this is the pure Chan of Tathagata."
(Mazu Daoyi, X1321p3b23; tr. Jinhua Jia)
“Don’t rashly seek the true Buddha;
True Buddha can’t be found.
Does marvelous nature and spirit
Need tempering or refinement?
Mind is this mind carefree;
This face, the face at birth."
(Nanyue Mingzan: Enjoying the Way, tr. Jeff Shore; T2076p461b24-26)