It was wrong. Turning up quick information makes it clear that hardtack was a core part of the military marching diet, and the soldiers could prepare their own bread, though they didn't carry flour.
https://imperiumromanum.pl/en/roman-army/food-in-roman-army/
https://www.comitatus.net/research_files/rations.pdf
Those both seem like decent writeups which cite to several classical sources. The first one mentions porridge (stating that bread was preferred); the second explicitly makes the assumption that all grain becomes bread. We can safely ignore that assumption.
More from that first writeup:
> The daily grain ration (wheat or barley)for one legionary was on average 830 grams and was in the form of unground grain; This was mainly due to the fact that grain spoiled slower than flour.
> The meals of the Roman legionaries were mainly based on wheat, from which two types of food were made:
> mash, called pulse. It was an easier-to-make food that required a mixture of cooked wheat grains, water, salt, fat, and olive oil or milk. Sometimes vegetables or spices were added.
> flatbread (pane). It was the more popular use of grain. The soldiers threshed out the grain, ground it with a device or stone, mixed it with water, salt and spices, and then roasted it over the fire.
So, what's true:
- The legion isn't going to carry a millstone. They're huge and there's no way to justify it.
- The legion isn't going to make itself dependent on the location of local mills. That would defeat the purpose of being a military force.
- The legion isn't going to carry flour, because it spoils. They carry raw grain.
- But they also carry hardtack, which is prepared so as not to spoil quickly.
- A foraging legion already takes raw grain. That's their primary foraging objective. They are not limited to robbing granaries; they're happy to harvest it directly from the field.
- Food preparation is done at a very low level - if the grain is going to be ground, it will be done in parallel by a large number of soldiers personally grinding small amounts of grain each, not by a mill.
- The soldiers often do eat bread. But if you're going to eat bread, you make it from raw grain the same day. This has to occur wherever you happen to be, so you'll be using something like a mortar and pestle.
- Despite the fact that bread is preferred to porridge, sometimes you make porridge anyway. It's much easier to make.