If the quote instead concluded with "The Marines verifiably know they do make a difference." 

Then it would say the same thing but say it poorly. That this ending even occurred to you as you read the quote should be treated as evidence that this is what was meant. It is a common and highly effective rhetorical technique to leave a key element implied rather than explicitly stated. Stating it explicitly undermines the rhetorical impact. We might compare it to explaining a joke, which kills the joke. 

But as it stands the quote just says the Marines no longer wonder about this, and presents it for a good thing.

You're misreading the quote by assuming that it did not leave a key element implied.