Guy thinks date is relationship

I posted a while ago about a guy I liked but didn’t really know that well. I had a mutual friend message him saying I liked him and why and that I’d go out with him if he was interested. All she said was I’d go on a date essentially. He goes and messages me saying he’s not interested in a relationship that it was sweet of me but he’s not interested. Since when did one date with somone become a whole a** relationship? Is this immaturity showing up? One date to me is one date to get to know someone and see if you’re interested, not an entire freaking relationship.