I think there is no lowering FS effect.It is also not an isobaric configuration.It is just for cancelling harmonics and reduce unwanted vibrations of subwoofer box.And controlled by DSP to give a flatter frequency response.
A tight slot makes "loading" in nearfield measurement. This ELAC sub will obviously sit on the floor and then lower woofer will get some extra floor gain, but nothing special considering room response in a normal size room.