IAU is well known for coming up with shitty arbitrary classifications about nomenclature that many astronomers don't agree with. They are wrong here because they don't take into account post-Cassini/Juno understanding of gas giant morphology. The IAU definition is outdated and highly misleading.

Copied from another reply I gave in this thread:

I've seen 13 MJ argued as a boundary, but it's selected somewhat arbitrarily and based around idealized models of Deuterium fusion, which has never been observed, and which is a process these brown dwarves would only undergo for a brief flash in their early life. Deuterium isn't abundant enough for its fusion to significantly alter the stellar morphology that has already become established for objects larger than Saturn. Saturn is our solarsystem's example of an object that does not fit cleanly into one side or the other of a mass-based binary classification scheme for determining a hard boundary between "planet" and "star". To understand what is a planet vs what is a star, study Saturn.