Shane was not sure how to properly answer the question about his soul world's safety.
Not only would his soul world be destroyed with his death, but there could also be accidents and intentional attacks from those who couldn't enter the soul world.
It may be unlikely for other humans to attack the soul world, but it was definitely not possible to leave out the possibility.
Because of this, Shane decided that it was best to be straightforward, and tell nothing but the truth.
"To be honest, I cannot promise anything about my soul world's safety, but I'm giving you guys a chance to survive right now.
Even if I wouldn't devour the secret realm, what would happen to it? The entire secret realm would be burned down, or if it was possible to somehow fight back against the fire spirits, all of you would have been forced to stay inside the secret realm, forever.