If you see the pictures of inside the DT1990 it could almost be classified as semi-open- although the design on the earcup looks like the whole back of the earcup is perforated like a typical open-back , IIRC the inside of the cup is just a small circle of holes in the centre, it wouldn't seem to allow significantly more free air movement than some designs I have seen called semi-open at least.
I am curious what they woudl do with DT1880... I have read the 770/880/990 variations all used the same driver, though not sure if that is actually true or not.