I was really referring to events on the Saxon Shore in Gaul but your point would hold true for Britain. We don't know who 'Saxons' are as it is a gallo roman term used to describe any germanic speaker operating in the north sea who is not a Frank. Even in the 8th century, people like Widukind thought of himself as a king of the Westfali rather than as a saxon. One 5th century gallic source refers to 'jutish saxons' around, what is now, the Antwerp area. The term saxon was, at the time, used rather like we use the term viking, with no distinction as to whether they came from Norway, Denmark or Sweden. I take the view that the term Litus Saxonicum refers to defences against the 'Saxons', even if it is partially manned by other germanics.This is a possibility of the situation in the late fourth century. It does, however, prompt the question where were these Germanic auxiliaries or federates from? They almost certainly would not have been Saxons garrisoning the Saxon Shore fortifications.