Click through to actually ordering, and when you select your OS, you're given a choice of only Fedora/Centos/Debian/Ubuntu/Windows Server. When I was looking for a host that supported Gentoo I found similar false advertising from them (and many others), they don't actually do it. In theory some of them will give you a recovery shell so you can then bootstrap whatever distro you want, but once you do that you're in very unsupported territory.

I use archlinux on OVH, so in this case the advertising is certainly not completely false.