“Thor was making a joke, as he is prone to do, when he said Groot was an elective.
But this theory contradicts the scene in Avengers: Infinity War, where Thor reveals he can understand Groot because he took his language as an elective course on Asgard.
Being a god of sorts he doesn’t have the same high bar for understanding Groot as the Guardians do.
Thor and Groot will likely reunite on the big screen for the third time in the next Guardians of the Galaxy feature, which Gunn is helming.