Quote Originally Posted by QwertyMIDX
It's actually more a propensity of modern historians. The roman authors usually say "armed in the roman manner" which we read as 'imitation legionaries' but seems to refer to, basically as you said, fairly well armored troops (most often mail), with large shields, who didn't fight in a phalanx (with the caveat that this almost always done with respect to hellenisitc forces). The EB team's stance has long been that when roman authors make the claim that some troops are "armed in the roman manner" we should treat it very caustiously and that it most likely refers to troops fighting as thorakitai or in some other celtic influence manner (i.e wearing mail).

EDIT: It is important to note that it doesn't often if even mean fighting in a maniple or using a gladius.
There are a number of factors which make me think that Polybius knows what he is talking about. He was a Greek man who had experience both in a Greek army (that of the Achaean league) and observing the Roman army in action (on campaign with Scipio Aemilianus), but he was probably writing for a Greek audience. He is also often very careful, as I said previously, in his classification of troops- he makes a distinction between thorakitai and thureophoroi, for instance. Polybius, of all the writers of Hellenistic history, would know what "armed in the Roman manner" meant, considering his experience with the Roman army. Considering all this, and the fact that Polybius only once calls any unit in the Seleucid army "armed in the Roman manner," I think that he specifically meant a unit modelled after the Roman legionary. So, knowing that the Seleucids had had a few nasty runins with legionaries, and that less than a half a century later they were employing men wearing chainmail and bronze helmets at Beith Zacharia and a unit which was called "Roman" by a man who would have been very familiar both with thureophoroi and thorakitai and the Roman legionary, it follows logically that these were imitation legionaries.