The cutaway shows the x-over/amps etc... on the rear wall, so it is not open.
I agree it is not open.
That design only makes sense if it is a coupled cavity isobarik speaker.
Therefore, they have sacrificed efficiency for a smaller cabinet.
The other issue is the bass augmentation bandwidth.
The problem with this arrangement, is that if you choose high bandwidth then you have high Q, if you want low Q then you have a narrow bandwidth.
Knowing Paradigm it is high Q. Their speakers always sound high Q to me and I don't think much of them.
The other issue is that this bass section is driven from the speaker level input. That is way short of state of the art.
To be honest, if I was given and unlimited budget and came up with this I would not own up to it.
The more I think about this design, the less impressed I am.
It really is just a similar version of these speakers are deigned and built 20 years ago, slightly over in fact.
The reason I built them was to have a location monitor that could reach below 30 Hz and be very accurate for monitoring recordings.
The difference is that none of the bass speakers radiate into the room, all speaker radiation is from the port.
The bass section is not driven form the speaker input.
It uses two bass mids of wide bandwidth. These drivers are in their own sealed compartment and have a frequency range of 90 Hz to 3 KHz. This speaker therefore has excellent power handling though the mid band.
Q of the isobarik bass section is an acceptable ( Just) 0.7.
So there are 5 drivers in each cabinet, but you only see three. These speakers where not designed as a last word design, but for a specific task, which they fulfilled admirably.