Cooling plates are used by NASA in the ISS; I've worked with that and many other applications in the DoD environment..
They can be very efficient, conduct heat away over large areas consistently, and while physically adding some volume, aren't terrible, but do add weight.
They wouldn't add too much depth to the pack for a top or a bottom cooling setup. The batteries at 86mm are only ~3.4in and the pack is well more.
And, they can serve as a ground plane for the vehicle if just used on the negative side of the battery, just as the chassis on every vehicle is today.
However, we have no evidence that cooling plates are an integral part of the structural battery pack.
And I'd guess Battery Day would have mentioned their use if part of the plan.