According to Theravada tradition, was Siddharta Gautama illuminated after meditating under the Bodhi tree?

I just found out what seems to me as a contradiction under Theravada tradition: When Siddharta Gautama meditated under the Bodhi tree, I understand ho achieved illumination, he reached the Nirvana. But Theravada tradition says that one cannot reach Nirvana while the five aggregates exist, since them are *dukkha*. Theravada traditions says that when one reaches Nirvana, one ceases to exist. Obviously, Siddharta Gautama did not cease to exist after the meditation under the Bodhi tree. My understanding is that after that meditation, Siddharta Gautama reached full understanding of the Samsara, and the path to liberation, but chose not to reach illumination, because he wanted to share what he learned. Is that the case? Or how otherwise do Theravada explain that Siddharta Gautama did not cease to exist after the meditation under the Bodhi tree?