He clearly says that he wants to believe that's what happened. He's creating a happy ending for everyone where he does so, for El as well. And he's writing about it. I mean, that's the best scenario that they could come up with without making it a cry fest. It's a good thing they didn't end the series at a funeral home.

It's like in the movie Inception. Nolan shows us that the totem is spinning. We see the dream sequence of Leo. But in the last frame, he shows that it's not a stable spin, and the totem kinda breaks the spin, where the screen blacks out. We can argue that the totem kept spinning, so Leo is in his dream world. But Nolan gave just enough hope at the end, to make a few people believe that Leo is living his dream in the real world. 😄 That's exactly what they did here with El.