They did, in fact, retreat to a nearby town named Placentia following the breakthrough of the Gallic infantry, where, according to Polybius/Livy, they would construct a fortified encampment. The reasons for this retreat is somewhat debated, but, it's not because they thought they won. Polybius, Appian, and Livy all have somewhat different accounts of the occurrences during the battle, so I think it’s necessary to present all of them. However, to acknowledge the point made in Historia Civilis’ video, I’d say it is misguided at best to merely speculate upon the reasons for the retreat here. He doesn’t even cite his sources until around eight videos ago, and, unfortunately, this video is among those not cited. Thus, I do not believe we will know his motivation for saying that without a comment from him.

Let’s start with Appian. Unfortunately, Appian’s writings fluctuated in quality. When he uses Polybius on The Third Punic War, it’s phenomenal. When he uses other sources, it turns to absolute garbage. He even goes so far as to say that Scipio and Hannibal had one-on-one combat during The Battle of Zama. Although Romans loved their one-on-one combat, it is frankly just absurd to say that generals would have fought each other, risking their lives in the process. Hans Delbrück, amongst the first military historians, even goes so far as to quote Appian at great lengths to show how bad his account of The Second Punic War is. Also, to be fair, he has no military experience to his name.

Appian’s statement regarding the retreat didn’t even go to Placentia originally, but instead towards the fortified encampment.

“Fearing lest they should be surrounded, they everywhere broke in flight to their own camp. Many foot-soldiers were cut off by the enemy's horse and many perished in the swift stream, for the river was now swollen with melting snow so that they could not wade, on account of its depth, nor could they swim, on account of the weight of their armor. Scipio, who followed trying to rally them, was wounded and almost killed, and was with difficulty rescued and carried to Cremona. There was a small arsenal near Placentia which Hannibal laid siege to, where he lost 400 men and was himself wounded. And now they all went into winter quarters, Scipio in Cremona and Placentia, and Hannibal on the Po.”

It’s not entirely clear whether or not the ‘arsenal’ Appian mentions is made up of those who retreated from Trebia or were there before the battle, acting as a garrison. After all, Scipio did winter there, and may have planned on wintering there. However, even if it were those who retreated, the remaining troops wouldn’t have amounted to just ‘a small arsenal.’ In my opinion, this theory isn’t very reliable. I think the next two are the most usable, but they don’t come without their problems.

Let’s talk Livy next. He’s usually regarded as a fiercely motivated conservative patriot, so some information may be somewhat fibbed in favor of the Romans. However, he is a very good source when it comes to the details of the battle, but, like Appian, he had no military experience- so some information may become muddled.

We do know that the weather was not good in any sense at Trebia during the battle, so Livy’s account could be accurate. He states:

“Romans experienced a fresh alarm when they saw their auxiliaries routed. And so, hemmed in as they now were on every side, about ten thousand men, when they found it impossible to escape at any other point, forced a passage, with great slaughter of their enemies, through the Carthaginian centre, which was composed of Gallic auxiliaries, and being cut off by the river from returning to their camp and so blinded by the rain that they could not well discern where to help their comrades, took the shortest way to Placentia.”

Again, it’s possible the rain did blind them into not being able to see them, so they retreat to Placentia. So, I will cede some ground when it comes to the ‘victory theory’ here, due to the fact that the Romans may not have been able to see their allies being encircled due to the terrible weather.

Now, let’s talk Polybius! From a very early age, he was disposed to military. He was even elected Hipparchus, a cavalry officer. Eventually, he was given over to the Romans as a hostage. Due to his background, he tutored Scipio Aemilianus, (adopted son of the eldest son of Scipio Africanus.) the Roman general who would later go on to sack Carthage. Eventually, he would accompany him to that battle, as well. However, Polybius states another likely reason for retreat:

“Just then the Numidians, who had been lying in ambush, left their hiding-place, and by a sudden charge on the centre of the Roman rear produced great confusion and alarm throughout the army. Finally, both the Roman wings, being hard pressed in front by the elephants, and on both flanks by the light-armed troops of the enemy, gave way, and in their flight were forced upon the river behind them. After this, while the centre of the Roman rear was losing heavily, and suffering severely from the attack of the Numidian ambuscade, their front, thus driven to bay, defeated the Celts and a division of Africans, and, after killing a large number of them, succeeded in cutting their way through the Carthaginian line. Then seeing that their wings had been forced off their ground, they gave up all hope of relieving them or getting back to their camp, partly because of the number of the enemy's cavalry, and partly because they were hindered by the river and the pelting storm of rain which was pouring down upon their heads.They therefore closed their ranks, and made their way safely to Placentia, to the number of ten thousand. ”

Thus, it’s appropriate to at least say that the Roman main line was convinced that defeat was imminent, and retreat necessary. What you believe, though, is up to you! Please reply with any further questions you may have, and I’ll try to get to them as soon as possible.

(Wrote this in a hospital bed with my laptop, so I apologize if there are any blatant spelling/grammar issues.)

EDIT: Did some grammar cleanup.

Sources

Hans Delbrück, Warfare in Antiquity

Adrian Goldsworthy, the Punic Wars

Robert L. O’Connell, The Ghosts of Cannae

Polybius, Histories

Appian, War Against Hannibal

Titus Livius, The History of Rome, book 21