It's like all that speech about understanding another country's customs went through the window. I mean, no one told him about the meaning of gifting a magic stone to a woman, but here he was explicitly told those cups are typically meant for couples. It's not quite denseness, but he comes off as almost insensitive this time, like he can only think of work; then again, he doesn't want to disappoint the Demon King, but damn.
Also, Ulmander being a tsundere does not help. She just can't make it clear she likes him, and being different species means he probably can't imagine she could. She won't go anywhere until/unless she manages to break his 'work first' mindset.