Good point
"One of the upright partitions dividing a ship into compartments and serving to add structural rigidity and to prevent the spread of leakage or fire."

They are being used for structural rigidity but probably won't do much to prevent the spread of fire.

Why Ply? Probably because I don't know any better. Although, I am in North America, I feel more isolated than you are because I don't have the Euro Tornado campaign experience that you have. Shipping foam, etc still has its complications. It is a marine ply and what was available to me. I glassed it on both sides and sealed the cuts with epoxy. Without spin-cat competition to drive me on, I feel my skills have plateaued and a few grams are moot. Around the cans, I finish 6 minutes ahead of the next competitor, how can I tell when I'm slow?

On the other hand, when I first brought this here, a place frequented by designers, builders and manufacturer's, no one suggested different. You mentioned taping the joints and putting holes into the "beams". I did do both although I admit I didn't go for the full height "beams". Shaping and fitting each was a bit more than I wanted to take on.
Or would they have become bulkheads even if they didn't stop smoke and fire?