Ah, you are right! Page 82:
"Note that, as a vehicle, the Chariot (and its rider, whilst embarked) cannot be locked in combat." I haven't run into too many Chariots so I made the (bad) assumption that they work like Walkers, which
can be "locked" in combat despite being vehicles (Page 84).
So as a correction to what I said before:
General_Chaos could have fired Overwatch at the second unit, but only because the first unit was a Chariot. If the first unit was a Walker or non-Vehicle unit he would have been unable to still.