[heja2009]

Well, for rooms with actual tatami mats, you are not really interested in the area, but rather how many mats will fit. Thus, a 24m^2 (12 mat) room with the wrong aspect ratio will not fit 12 mats. (I'm using the 1 mat = 2m^2 conversion for convenience and I actually use 2m^2 mats in my tatami room.) Practically, mat sizes can vary a bit and can be custom sized. [1]

Even though today only a minority of apartment rooms are actually used with tatami it is still customary to give all rooms in mat units and usually in m^2 too.

BTW that's similar to how non SI units are still used in other countries nowadays, e.g. (metric) pounds and half-pounds are still used in context of groceries in colloquial German.

[1] https://en.wikipedia.org/wiki/Tatami#size