For readers from outside the Buddhist context, it's worth noting that the above comment is written from an entirely Mahāyāna perspective. For example, it isn't true that "every Buddhist tradition agrees that all living beings possess Buddha nature," that's a very specifically Mahāyāna idea that arose centuries after the death of the mendicant Gotama who is known as the Buddha.
The concept of "religion" as it is meant in the modern context is only a few centuries old, as is the idea of "Buddhism", but to the extent that we can think of the teachings of the Buddha as a religion, it's probably most helpful to think of Mahāyāna and Theravāda as different "religions." I frankly think that the Vajrayāna subset of Mahāyāna probably is best thought of as a different "religion" as well, but it's not my area and I wouldn't suggest anyone take my thoughts on that topic with any great heft.