Wouldn't it be about the bass extension each version has instead of bass quantity?
Usually open-back headphones have a well extended bass, which gives a depth sound to it.
In contrast, closed-back retains the bass thump in the cans and reflects it back to your ears, making it feel like the bass in a studio with isolated walls rather than on a stage or in an open hall.
The semi-open would be sonically situated in between, with its precise location depending on how open/closed they are (some semi-open have smaller/narrower "vents", resulting in a sound closer to a closed-back, whereas other have some pretty large "vents", sounding more like an open-back.
For comparison, the Superlux HD668B (semi-open) sounds more open than closed, in my opinion.
I'm not sure why they say that the open-back has more bass than the closed-back, though.