The MBTI is descriptive. However, it claims to be descriptive of internal mental processes, which it posits to exist in a certain way (admittedly not everything interprets the MBTI this way, but IMO this is the only interpretation under which the MBTI is an interesting theory).

I'm personally quite open to the idea that it's an accurate categorisation, but I do also think that it's an extroadinary claim that requires extroadinary justification. IMO saying "oh, it's just a description" because that evidence hasn't (yet) been found is a bit of a cop out.