The AI does in fact have an idea about armies, and holds up to (i think) 4 of them at any time. This does depend on how many asteroids that AI owns though. At the start they don't have any armies. What they do is send a bunch of their own seedlings to wherever their current army or armies are, or move the armies around from one place to another.
What the AI doesn't do is any kind of learning at all - it's much easier to do things like examining targets and deciding how many seedlings to send / whether to send seedlings at all, than to implement learning from mistakes, although that would be much cooler.