I would assume that it means that the kernel, the actual operating system core itself, is a modified Ubuntu kernel, while the distro itself is CentOS. That would be kind of odd since usually the kernel is built as part of the distro so all the management tools and utilities are built with the same libraries and such, though, but I guess it's feasible.
It could be that the tech was just mistaken, or someone told him that they made some customizations to the kernel to add a feature like Ubuntu's default kernel has. You should be able to recompile a kernel to customize it (add/subtract support for features that are unneeded, maybe make the kernel smaller or reduce memory footprint or add support for something in their cloud that the default doesn't have) and so he just got used to saying that when customers called.
The advantage disadvantage question is moot without knowing what alterations were made or customized. If I had to guess, I'd say it isn't an ubuntu kernel, as I'm not entirely sure you can just drop it in as a replacement. 99.9% of people out there replacing the kernel would do so with either a repository alteration from the distro or through a recompile with their distro's toolchain.