As you likely know, within the vented subwoofer type, there are several different variations.
LLT; large low tune
SLLT; super large low tune
An IB, utilizing an attic, crawlspace or similar area as the backspace is typical. However when one designs a tuned port to augment the response (at the expense of all response below tune), a name of ported IB seems appropriate, regardless of technical accuracy in my opinion. When you see it, it's a ported IB. All that said, you're right, it is ported and SLLT is most likely the correct designation.
. This is an outstanding explanation by guru Steve Callas.
LLT vs. SLLT explanation
The individual that transformed his IB into a SLLT, wanted more impact, feel etc. Oftentimes the IB alignment, is so transparent and clean, that the dramatic reduction in distortion, isn't overwhelming in the short term. The high resolution, transparent nature is more more pleasing over time. Without a peaky response, either inherent or EQ'd in, an IB allows the material throughput to be rendered cleanly, and without un-natural emphasis. This critically damped, low Q response isn't of the "blow you away" variety. However, it doesn't wear thin over time either. Subwoofer subjective terms such as "slam", "impact" etc., are merely descriptors of frequency response characteristics. If the ported IB gave the owner what he desired, that's great. However, below tune, with any
vented design, response drops off in a big way, and distortions rise dramatically.
Many contemporary motion picture releases recognize the realism of including infra-sound content down into the low single digits, and this energy helps create a very realistic suspension of disbelief. So if system response down to the 3hz (to cover the LFE specification of 3-120hz), is the desired outcome, even the best vented design can not get you there.
Back OT; I'm looking forward to you integrating and optimizing your IB, and of course your impressions of the IB system.