Well he does have the stolen vessels. Such as the Centurion and Interdictors, but that really doesn't matter when you have a good chunk of your navy being categorized as cannon fodder. Which is essentially what the Derriphan has become.
Wow that is a terrible ship...no shielding? A class 4.0 hyperdrive with a backup 12.0? The only thing good about it, is the concussion missiles but even then, Xizor's ships can be outfitted with point defense laser cannons(in fact some of them already come with those). If thats what a good portion of Revan's fleet is then...wow.
During Naga Sadow's time, the Derriphan was akin to the Star Destroyer. But now that technology has advanced so far, it can barely fulfill the role of light frigate. It's really not that good anymore.