What I think they're saying is that they only test on CentOS and Ubuntu, two significantly different flavors of distro. I don't think they're saying that it won't build on fedora or debian.
The idea that they should test on every distro in order to say that it works on Linux is a bit silly.
Through the eyes of the author, the original comment probably seems obvious in what it's attempting to convey. Through the eyes of other people, its intent can be ambiguous. In my case, very ambiguous. I wouldn't be surprised if you were downvoted just as much for being unclear as for people misunderstanding your point.
What I meant is that they don't even say 'works on Linux' like in 'they don't even cover all the most commonly deployed Linux distros'.