I would argue that science is about a core of what is considered proven and thus is true, as well as topics that are currently discussed and a large amount of knowledge that still has to be discovered. And I think these galaxies that are invisible today might be in the latter category - if they exist at all. But I wouldn't rule out their existence unnecessarily. In a few years, due to advances in technology, it could be possible to "discover" (or rather estimate) far more galaxies than what is proposed in this study.

Just because mankind in the middle ages had never seen Pluto doesn't mean it didn't exist back then.