Cladistics. A discovery can be shown to be representative of an evolutionary "link" without having to be demonstrated to be "the" link itself.
Actually, scientists are well aware of the issues you raised, and generally use language that is more consistent the understanding that such "links" are more likely representative offshoots of actual common ancestors than "the" common ancestors themselves. Note that Moya says "a common ancestor" rather than "the" common ancestor. And the research papers are even more careful in how they describe such things.
But press accounts of such scientific discoveries are often "dumbed down" (either through oversimplification, or the reporter's lack of understanding) and frequently state things in more simple -- but inaccurate -- terms.
Always check out the actual research papers, and take with a grain of salt (or a whole bag) whatever a "pop science" article has to say about it.
Garbled articles for the public about scientific issues is a pet peeve of mine.
Dianetics. A discovery can be shown to be representative of a "link" to Xenu without having to be demonstrated to be "the" link itself.