Buddhahood and buddha-nature are not the same thing and the distinction is easy to lose. Buddha-nature, tathagatagarbha, is what is said to be already present in beings; buddhahood is its unobstructed expression. The Ratnagotravibhaga sets out the classic account with a series of images: gold buried in filth, a treasure under a poor man house, an embryo in a womb, all of them things whose value is complete and merely hidden.
That doctrine created a difficulty the tradition argued about for centuries. If the nature is already present in everyone, then everyone will become a buddha, and the Lotus Sutra one vehicle teaching says exactly that. But some Yogacara texts sort beings into families, with those destined for the hearer path, the solitary buddha path and the bodhisattva path, and a class of icchantikas, beings said to be cut off from awakening altogether. The two positions cannot both stand as written.
East Asian Buddhism resolved it decisively in favour of universality, and the position that all sentient beings possess buddha-nature became standard there, with some Japanese schools extending it to plants and to the insentient world. Tibetan traditions kept arguing, and the dispute between reading the nature as empty of its own nature and reading it as empty only of what is extrinsic to it, the rangtong and shentong positions, remains a live division between schools rather than a settled question.