I take it you mean the theory about
Totoro being a god of death and Satsuki and Mei being thought to have died etc? I read up on it just now and it does seem slightly credible, but honestly I think the general concept is all too far-fetched for me personally to accept, especially when the creatures look so happy and the film seems so light-hearted.
That theory is so far off the road that it's not funny.
It's a modern riff on a Shinto folklore. Every creature in the film represents a component of the nature, which is a basis of traditional/folk children's stories.
But I'm responding to an old post, so I shouldn't elaborate on MFT any further, in case it may bore your pants off, but I am happy to explain if there's still interest.