Technology is a game changer. If Lee had nothing more added than just radio communications to coordinate his attacks at Gettysburg, he wins.
No, a single Marine unit couldn’t slay mass numbers of Roman troops without resupply, but Hernando Cortez defeated an entire Inca Empire with less men than what is being discussed here.
That is the wild card. The technology the Romans encountered by a Marine unit would make the Romans believe they were being punished by the Gods and would more than likely just hand the Empire over to them. Huge metal rooms that move and shoot and explode things. Metal sticks that launch deadly projectiles. Even the radios alone would freak out the Romans.
So no, the MEU could not defeat a Roman army, but they could certainly conquer the Roman empire if they were mistaken for gods.
Hernando Cortez burned his ships to make a point with his men that it was conquer or die. Do you really see a Marine Colonel going all-in like that? Modern values kind of mitigate against wholesale slaughter. Even if he were to give such an order the Marines might hesitate to engage in wanton killing. Unless of course everybody "got their mind around the situation". (Realized that they had little choice).
Not just tech as the game changer.
Dietary changes over the period would also have its impact. Modern folk are taller, bigger and heavier muscled than ancient folk. That, taken along with advances in medicine and such, modern folk would look like gods to the ancients.