I guess he was wearing shoes so there was just nothing they could do to stop him.

It's the upgraded level 10 (?) skill option

Yep, my Achilles is doing that too. Just running him around, mowing everyone down. AI in a bid to get away from him, run piecemeal out of the settlement into my waiting army.

Not sure if it works for all heroes or just Achilles, but Achilles has soloed 4 settlements already in my campaign.

It also gets you like 2 level ups per fight, something like 2200 EXP

[Troy] PSA: Achilles with a reinforced chariot can solo autoressolve settlements for a heroic victory