Two often misunderstood terms come to mind.
You could say that the idiom begs the question:
To beg a question means to assume the conclusion of an argument—a type of circular reasoning. This is an informal fallacy, in which an arguer includes the conclusion to be proven within a premise of the argument, often in an indirect way such that its presence within the premise is hidden or at least not easily apparent.
People often use "begs the question" to mean "raises the question", but its original meaning was including an assumption in a statement about that assumption- such as the fact that geese are easily scared.
You might also call the idiom a factoid:
The term factoid can in common usage mean either a false or spurious
statement presented as a fact, as well as (according to Merriam
Webster and the Oxford English Dictionary) a true, if brief or trivial
item of news or information. The term was coined originally in 1973 as
a neologism by American writer Norman Mailer to mean a "piece of
information that becomes accepted as a fact even though it’s not
actually true, or an invented fact believed to be true because it
appears in print." Since its creation in 1973 the term has evolved
from its original meaning, in common usage, and has assumed other
meanings, particularly being used to describe a brief or trivial item
of news or information. So it is a factoid that "factoid" means
something that is true.
People often use "factoid" to mean "a small fact", but its original meaning was a false statement that was repeated so often that people thought it was true- such as the fact that geese are easily scared.