I have a lot of faith in the collective knowlege here. Hopefully someone can come through with more than a guess.
I was in Wisconsin Wed-Sat and while touring a firetruck manufacuring plant we had this question. How is diamond plate made?
The back side is smooth, so it's not punched from the back.
It is made out of both steel and aluminum.
While aluminum diamond plate is widely used in this factory, it is not made there and the workers I asked didn't know how it is made.
We made a few guesses and I may be close to the truth, but I don't KNOW. My best guess is that it's forged.
I'll wait for some of you aluminum gurus to check in...