Trailers have pass-thru cables so the loco and the cab car can communicate on opposite ends when the train is in push mode, but the trailers themselves are completely inert in terms of train control. They simply have the electrical hookup for hotel power, the air brake line, and the data cables for PA/PSA, automatic door controls, etc. In an MU consist, all of the cars--including unpowered trailers--talk to each other 2-way in order to coordinate propulsion on any power car that may be positioned anywhere in the consist. The trailers also talk, because unlike a loco-hauled consist where they're an unknown inert mass (e.g. the loco or cab car doesn't care if there's a mixed set of new bi-levels and ancient flats...or how many of each there are) the trailer has to provide data about its properties in order for the MU trainlining to smooth out the ride. That's why unpowered trailers are nearly always manufactured to match their make/model of powered MU's, including and off-shelf coaches are never coupled to an MU (be it an M#, Arrow, Silverliner, ACMU/MP75, RDC, Colorado Railcar, etc.). At least with the RDC, you'd violate the Budd warranty if you ran an RDC coupled with an unpowered any-car because of the excessive wear it would cause.
So with this order there's still something undisclosed about how they plan to do this. The original plan was to do power cars with off-shelf MLV coaches using the same passive trailer connection (2 consecutive trailers for every 1 adjacent power car), but because that forces the power cars to overcorrect for the inert mass the power cars would've been overpowered, extremely complex at the computer level, and heavy. So NJT keeps saying this is not that plan, that this is some form of 'true' MU. If it's a true MU, then it has to have a true MU data connection where the unpowered trailers are capable of active 2-way communication unlike an off-shelf trailer. So the question is how they plan to do that. The most logical solution would be to run 2 types of trainlining connections through an otherwise vanilla MLV trailer: 1 for operating in 'active'-communication MU mode, 1 for operating in regular 'passive'-communication loco-lauled mode. And maybe for 'active' mode there's a small computer onboard just identifying the car's properties...and making sure that they DON'T plug something incompatible like a Comet into the MU set. If BBD's design is clean enough and tests out well enough, you should be able to mass-manufacture fully stock MLV III's with both sets and make them totally universal cars. And possibly re-wire the MLV I & II fleets for it, though that would be a little tougher correcting for evolutionary differences between the batches.
But there's still a lot of detail lacking on exactly how they plan to do this. Things like that contradictory language about wanting the powered MU cars to trainline with the existing MLV fleet and not the next batch. That doesn't answer the "how" question of how these are going to have a true MU connection, and starts to sound a little more like the original rejected "power packs" idea...only they're still saying this is an MU and not a power pack. So we need more information because those little discrepancies are still a head-scratcher.
