How were the Sandersons able to survive being burned in the kiln/school furnace?

If this isn't a big time plothole then I don't know what is. Remember when Max, Dani, and Allison lure the Sandersons into the kiln and set them ablaze, presumably killing them?

https://www.youtube.com/watch?v=VY7nMmxLu6E

How come the Sandersons don't really die? Unless I'm missing something, the black candle was only able to bring them back for a single night. To clarify, the movie as far as I know or can recall, didn't say anything specific about the candle making them impervious to pain or protecting them from any injuries or harm. In other words, it doesn't like Binx, make them immortal.

If that was the case, then how come back in 1693, the Sandersons meet their first and initial demise by being hanged? That to me, proves that they are capable or susceptible to being hurt or killed.

I've read the argument that the Sandersons are technically, already ghosts by the time that they come back in 1993 and are bound by the candle. Therefore, they can only be on the mortal plane until sunrise.

https://www.reddit.com/r/AskScienceFiction/comments/jm3t6a/comment/gasri5o/