Confirmed: How the AI "Cheats"
After another suspicious rush on week 2, I downloaded a cheat addon for BepInEx to watch the AI play and see how it behaves. Did they get lucky with Pandora's boxes or Artifacts? Do some fancy hero chaining?
Nope!
How the AI "cheats" is pretty simple: They can't lose units while fighting neutrals. Not a single one. For instance, I observed Week 1 Hive heroes run face first into archers and not lose a single unit. As anyone who's played Hive knows, that's not possible.
And before "they probably used magic or had some badass scroll" - nope! I inspected their starter areas. I observed the same behavior in 4 different AI Heroes. Not only were at least half of them Might heroes, Magic Heroes can rarely do a perfect clear in week one anyway. Not only are your spells too weak to wipe the bad guys yet or last long enough for you to crowd control, Archers will get shots off.
The AI doesn't just attack anything though - they do obey some kind of logic where their army has to be sufficiently strong before attacking a neutral stack. But since they never lose anything, their snowballing is literally perfect.
When you encounter the AI, their hero hasn't lost a single unit since the game started unless they moved them to a different hero or encountered an enemy hero. That's why they have heaps of artifacts and 2-3x the Tier 1 units you do: they can't suffer any incidental attrition from clearing.
With no attrition, they hit the critical mass of units required to break pretty quickly - and lose nothing in the break. Their next move is then to break into the following zone - which is sometimes the one that leads to your base.
Now, why do I put "cheats" in quotation marks? Because there's a pretty good chance this is a bug. There are way cleaner and much more believable ways to help your floundering AI than to simply disable losses in battle - that leads to unrealistic army sizes, like we're seeing now.
The theory this is a bug is also supported by the fact it happens on all difficulty levels. I could see this as a feature of Unfair, which is outright described as cheating, but Normal and Hard? It doesn't make sense there.
So yeah. As far as I've observed, the AI is absolutely cheating. I just kind of doubt it's actually intended to be cheating in this way.