Guess those two girls are airheads, and probably the reason the Demon King died in his previous life, as he had to protect them.
I mean, Demon King is probably an existence which would be considered ominous and fearful to the humans in general. So, why would you so openly call the guy out like that in open public space, endangering both his life and social position. Logically, they should have been that first talked to him when he was away from public, and then ask him to come with them and then introduced themselves and talked about his past.