> If Crazy Carl says that aliens abducted his prize turnip, that is in itself evidence that aliens did the deed. It is wildly unpersuasive evidence, but nonetheless evidence.
You're correct, evidence exists on a spectrum from "highly implausible" to "highly plausible".
> People like to believe that "someone says" isn't evidence, which is wildly disconnected from how human society works.
Human society is multi-faceted.
In the sciences, "someone says" is the lowest form of evidence there is. It is so close to "no evidence" that we simply disregard it without further thought.
In the legal system, "someone says" is the highest form of evidence there is - witness testimony can, and routinely does, trump any other evidence.
These are the two extremes. Other human societal systems use the word "evidence" to mean anything in between these two extremes.
You are conflating the science's usage of the word "evidence" with the legal usage of the word "evidence".