He's speaking about Arsenal p 132, Armor Modification.
This is purely for Armor
mods and would not effect anything that has a starting armor above 20.
That 20 cap is a slight misread though. That is saying the rating
of each can go up to 20 for normal, 10 for concealed or smart, not the armor value itself is capped. You can combine regular armor and smart armor for results above 20. They do it themselves in the next line about the Steal Lynx:
...
could be equipped with a maximum of Rating 10 concealed armor or Tating 12 normal armor + Rating 10 smart armor.
...
So you could have up to 20 normal + 10 smart. A total armor rating of 30.
While it doesn't specifically SAY that smart armor also adds to the armor rating in addition to it's AP negating effect, I
assume that it does. After all, it says that it is superior to normal. Additionally only putting smart armor on a vehicle would be fairly pointless otherwise and there appears to be nothing stopping you from doing that. And finally, the description for normal armor ALSO fails to mention that it adds anything to the vehicle's armor rating. Details

Anyhow, my take on it is that smart armor adds it's rating to the armor pool as well, at least so long as it still has uses. I could be wrong on this though.