I'm a big fan of Bret Deveraux and a long term subscriber, but this is mostly nit-picking.
The Roman armies didn't dress as uniformly as this? They did in contemporary depictions (Trajan's column) so this is only as much artistic license as real Roman artists would take, and for the same reason - it's more visually impressive.
A Roman army wouldn't have this mix of units, or at least, it would have them in different proportions? Well, this wasn't typical, but you fight with the army you have - maybe your light infantry took heavy losses in a previous battle but you were able to cobble together a load of extra archers.
The tactics weren't typically Roman - or maybe they were, but not in 180 AD, because that specific period happens to be well-documented? Mixing some patterns from different eras is again allowable artistic license, one of the things that makes this art rather than an exact reproduction of a specific battle. And also, a good general adapts to the conditions: he had more archers than typical, so needed a different battle plan.
Comparing the weaponry anachronisms to having tanks at the Battle of Gettysburg is unfair: the American Civil War lasted 5 years, while Roman campaigns in Western Europe (Gaul and Germania) lasted at least a few centuries. It's closer to having an ironclad show up at Fort Sumter and complaining that design wouldn't exist until a year later.
The title literally says it's nitpicking, so I'm fine with that. But the introduction oversells it a bit more: "such a deceptive historical mess", etc. Like I said, big fan, but I was expecting inconsistencies more on the scale of "they seem to have marched 30,000 troops 100 miles in two days to relieve Minas Tirith, with no sign of any logistics to supply them".
> but not in 180 AD, because that specific period happens to be well-documented?
As for "why pick 180 AD then?" it's clear that this was chosen in order to have Commodus as a villain. Commodus is shown mostly historically accurately: he really was with his aging father Marcus Aurelius in Germania, he really was a teenage emperor who was seen as immature, vain and capricious (if the historical record doesn't explicitly support "downright evil") and he really did love staging ever-greater gladiator games.
> They did in contemporary depictions (Trajan's column) so this is only as much artistic license as real Roman artists would take, and for the same reason - it's more visually impressive.
The point is that Trajan’s column was propaganda. So this depiction is second-hand fiction (rehashing ancient propaganda with modern values).
The whole point of the series is that the scene is completely wrong from a historical point of view.
> Well, this wasn't typical, but you fight with the army you have
He makes the point that this was way outside the normal composition of a Roman army, not slightly off. And that as a result it would be terrible for Roman tactics and would have no hope to survive such a battle. Also, medieval siege engines in a pitch battle is just silly, there is no defending that.
I enjoyed the movie but I think that we can completely drop the assumption that it has any grounding in historical reality. It’s fiction, just slightly more realistic than Games of Thrones. Then we can appreciate the movie for the silly entertainment it is (and he does not dispute that).