First off a reader needs to disconnect from our Earth's reality and connect with the alternate Earth's reality. What must be realized is that these people do not have our history of armoured warfare. In fact, without getting into spoilers, these people have very little military experience and it shows in their tactics and the equipment that they use. While it is true that the alternate Earth is a high tech world compared to our Earth, most of that high tech is focused on very specific technical fields and the production of armoured cars for riot control isn't one area that they're experts at. You could say that the armoured cars were an improvised solution that didn't work.
When I wrote the battle with the armoured cars I was visualizing the armoured cars of the interim years between WWI and WWII. Many of them were simply a truck chassis with armour plating attached and they could be easily taken out with well placed grenades or machine gun fire.
Hopefully this clears up any questions. Thanks for reading.