Have all our buddhas since Sakyamuni not abided in nirvana?

As far as I know, many practitioners in the mahayana tradition have claimed to reach buddhahood and nirvana, since Sakyamuni. Is this non-abiding in nirvana? It would explain how prominent Buddhists have in the past been identified with celestial bodhisattvas. If so, would they nevertheless have reached final nirvana, on death?