This seems to me to be an unreasonable interpretation.
Yes, the Western Empire was less urbanized, less populous, and possibly less rich than was the contemporary Eastern Empire. So what? The Western Empire was still an extremely large state that could mobilize hundreds of thousands of skilled fighting men and had millions of solidi in revenue every year. The East's riches were slightly greater than the West's, not enormously greater. If the ERE's yearly revenues were about 8 million solidi, the WRE's might have been around 6 or 7. (Educated guess on the first one, ass-pull on the second.)
By comparison, the Western Empire faced a relatively negligible number of "barbarians" who were invariably less well-armed than its own soldiers and, most importantly, were rarely organized well enough to be a serious military threat. The Emperor Iulianus made his name by defeating a force of "Alamanni", described as the most serious invasion of Gaul in decades, that numbered less than twenty thousand, and probably little more than ten thousand. This was not the stuff of imperial conquest. By comparison, the Sasanian Empire could raise multiple armies of twenty thousand at the same time without breaking a sweat. These armies were armed just as well as Rome's armies were, were quite well trained, and were led by the closest thing that the late antique world had to professional soldiers.
The military threat that the East faced was so disproportionately large compared to that the West faced that it is prima facie ludicrous to explain this gap via a minor revenue disparity and an incremental urbanization deficiency.