I like history too, and I would appreciate a card from Auschwitz. Such cards give me pause for reflection. It is important that such things are never forgotten.
I think some folks are here for happy vibes only? That’s ok too, I guess we are all here for our own reasons.
Based on the message they wrote (… terrified that someone would like to receive such a depressing postcard, that his/her good mood would instantly disappear when opening his/her mailbox and find a card with such a theme in it…), it seems the sender was unable to see past their own reactions and feelings. I am surprised they shared such a message with you though, instead of keeping it more simple and neutral with a message they were glad or relieved to have a recipient for that particular card.
I have a special stack of cards waiting for a recipient who will enjoy receiving them.