Everything is casual to Reborn, it's just as natural as breathing to him really. Don't be bothered, stay calm, don't show you care, things he's had drilled into him for years. And dropping that bombshell on her is normal to him.

Having a weapon pointed at him is too.

"They survive off of a different type of flame that I only recently learned of, one that needs hatred and comes only from their leader." They survive off of hatred and a desire to kill Checkerface, but he's not spilling that last tidbit, not yet.