It’d be pretty weird (or just funny) if Thor intentionally introduced his friend by the wrong name — unless Thor never learned Groot’s name, but that also seems implausible.
Posted on Reddit, one fan lays out his theory on why Tree is Groot’s real name.
In Infinity War, Thor introduces Groot to Captain America as Tree.
Furthermore, the Dwarf king Eitri also refers to Groot as Tree, but it makes more sense for Eitri not to know Groot and just call him by what he is.