That may be the case now, but ages ago it was just a fancy BSD box as far as I could tell. I used to be a reseller for their product circa 2000 and I wasn't aware of any proprietary hardware inside, just a really lean networking stack.
We were getting slammed on duty for the product and we were looking at ways of getting the appliances built locally using licensed F5 software, as the software itself wasn't as heavily taxed as the physical hardware. Everything in it seemed fairly commodity, except for the big F5 logo on the front.
It was an appliance that worked fantastically well. One deployment had an uptime of over ten years.
The data plane is where you have high speed logic and data traveling. That's where you do the multiple 100GBps software defined networking, and its crazy fast chips doing it.
And the control plane has interconnects to program the data plane chips to the rules you want. So the data never hits the control plane at all. Its kind of like a water faucet where the knob doesn't touch the water but controls the floodgates.