The way it's written now (The most powerful mobile electromagnetic railgun built by a non-government) seems pretty non-ambiguous in implying that more powerful mobile railguns have been built by governments.
I suspect that government defense contractors have repeatedly built man-portable prototypes that just don't measure up favorably against the .50-cal rifle. The government then mothballs the project again, until better electronics technology comes along. It also seems obvious that anything that did measure up would be classified as a state secret, so we wouldn't necessarily even know about it.
So the implication I got is that government-built devices are purposefully excluded, because we have no way of really knowing how powerful the most powerful government-built mobile railgun is.