This could be a way to tie him more into the plot and provide some exposition. Don't get me wrong, I am not arguing it's him for sure lol. Just think it's an interesting possibility, that will most likely not come true, and we will most likely find that out in 6 days lol.
Another interesting possibility for me is that imo here Falli implies humanity might have had weapons that could have dealt with the primal devils, or her specifically in the past, and those are most likely nuclear weapons