well, he's just been clearly established he's willing to lie out his ass when he thinks the situation/other person will benefit from it; he probably took one look at the walking stomach ulcer wearing another country's armor who'd just dropped in & decided it was best not to spill all the details lol. Especially if i'm correctly reading what the rest of bomb-girl's backstory set up- that being:
- Amon & his brother were orphans in the duchy of dumbass (wanted to think of a decent pejorative + clock pun combo but something like "Dick Tock" was as far as i got before aborting the line of thought from disgust with myself)
- they were picked up by the same training program
- (as we've seen) the training program involves all of the children fighting each other and the losers being disposed of; one of the two found out about this & realized they'd be pitted against each other specifically
- the two hatched an escape plan*
and then either only Amon escaped & he wandered until eventually getting eaten... or (what I feel is more likely tbh) the dragon magically sensed a couple of REALLY stressed out children and flew in for a snack the instant they escaped. (hell, maybe the dragon was the distraction they needed for their opportunity! they just didn't realize the fucker's like a stress-seeking missile who'd go after them specifically lol.) Amon's "cheerfully foolish" framing is probably because HE escaped from that life (directly into the dragon's gut lol) assuming they'd both get out, but his brother didn't- maybe he got captured at the last second or something- and ultimately brother blamed himself for Amon's (assumed) death, utterly broke mentally and went absolutely sicko mode, leading to where we are now
*i was considering maybe brother modified the warming magic into the bomb spell as a way to take the facility hostage in the process of escaping or something, but... that topic hasn't been expounded upon nearly enough to make that kind of assumption yet tbh, that could be basically unrelated to their interpersonal story & is just a "brother/whoever he taught it to kept on using it after he was gone & the psychos at the top thought of a really funny way to repurpose it" kind of deal or any number of other things