Frisk doesn't answer right away. They just look at Mammon, obviously considering something.

Their friends and family don't know everything about what happened to them. It's not that they take any particular pleasure in keeping secrets. If Flowey weren't involved they'd have told them everything, but...Flowey. And then there's the humans, for whom Frisk usually plays down the many, many attempts on their life by various monsters. If it were up to them they'd have lied about it entirely - none of that matters now - but some things still got out. Their very first diplomatic incident.

In the end, none of that really applies to strangers from another world. They're on the same team. Yeah, they conclude. They've seen worse.