The logic is fine, it's just massively overstated with the clickbait headline and travel time in the vicinity of an empire is an endogenous factor which depends on where the boundaries of the empire are drawn, because empires build roads, chop down forests, drain swamps and make rivers navigable.

As the article acknowledges, travel times within the Roman Empire tended to be less than a month. But they conquered land regions (where possible) before they built the roads that let them supply them that fast. Certainly there's nothing about lowland Germany that should make it more difficult to reach than Hadrian's Wall, but they effectively subdued the British tribes and built roads all over that particular faraway island, and didn't have as much success against the Germanic tribes who inhabited desirable land closer to their capital that could have been reached in a shorter time with a nice lowland road from Gaul, if they'd ever been able to build it.