>>279612225
The whole point of that story was for the Fire Devil to say that FSM lived to get revenge on Denji for his actions, to justify that whole situation.
But we know this isn't true because the reason FSM was there in the first place was because Death mindcontrolled him into doing so.
Before getting killed by Death, he was just trying to help random people and seemed more worried about the whole prophecy thing than anything else.