If languages are best learned from immersion, how is it possible to revitalize dead languages?

Through immersion.

Please read Daniel Ross’ answer and Jens Stengaard Larsen’s answer, which address the bulk of this.

The language you’re reviving is likely not going to be identical to the original language, as Jens points out; and that’s ok. I have a friend involved in language revival; she’s helping indigenous Australians reclaim their languages, and she’s careful to let them take the lead in the work (as you have to be). Because of both the dynamics of the situation (she’s not indigenous), and the fact that the community members are not professional linguists, she’s ended up skipping things like the ergative.

Yes. The ergative.

And that’s ok. The point after all is not to go back in a time machine and speak an identical language to that of the passed ancestors (even if that is the dream). The point is to revive something, and call it your own. And the most effective way to learn a language is still immersion, even if what you end up learning is not as historically sound.

Just as whatever got revived in the kibbutzim of Palestine was not a carbon copy of the Hebrew of King David: Hebrew had remained in use as a scholarly language, but there was plenty of Yiddish that got added to the mix, to get it speakable. The point was that the real revival of Hebrew happened in the kibbutzim, and not in the household of Eliezer Ben-Yehuda. There was immersion in both places; the immersion in the kibbutzim was less meticulous than what happened to Itamar Ben-Avi—but also more humane, and more scalable.

At least the kids growing up in the kibbutzim were allowed to have friends who didn’t speak Hebrew.

Population of Jerusalem speaking Hebrew when Itamar was a kid: 1.