This claim is from the introduction of Edward Luttwak's The Grand Strategy of the Roman Empire. The claim is unsourced and it is not expanded upon later in the book (as far as I'm aware). I don't see how that claim could be true; the Romans generally had similar weapons to their opponents, at least during their expansion under the Republic (the period I'm most familiar with).

Luttwak is a political scientist who wrote The Grand Strategy in 1976. His background permeates the book - that's not just my opinion, but something also recognised by Erich Gruen when he reviewed the book in 1978:

The political scientist's tendency to overrationalize will trouble historians who know that events are not always dictated by human calculation. Luttwak sees design everywhere.

and

So speaks the professional defense analyst.

Moreover, the ideas expressed in The Grand Strategy have been superseded by works by William Harris, Arthur Eckstein, and Erich Gruen published in the decades following the publication of The Grand Strategy.

In short, the book as a whole isn't that good (any more) and that particular claim from the book is unsourced and unsupported.