Armor is a philosophy, this robot is built around mobility (like a human) and delivering the firepower of several soldiers simultaneously with precision. Realistically it's not very hard to penetrate armor, infantry are capable of carrying anti tank weapons, and even modern rifles are now capable of using munitions which can penetrate light armored vehicles. A small bot like this is not capable of carrying enough armor to withstand penetration before it would be slowed down enough to be vulnerable to the next grade of anti armor weapons. Basically, this bot doesn't need heavy armor, because it will be avoiding weapons fire that it is vulnerable to.
I'm seeing a lot of inspiration from Ghost in the Shell. Vague images cross My mind from an episode where Batou and some chick where checking out an illegal mining complex, and when discovered, where chased by hazard suits looking very much like this.