When I challenge a statement and receive the reply "Oh, I don't know", I figure that the statement was not based on fact, since the issuer cannot bring forth a fact to justify the statement.
A non-fact is a supposition, a fabrication, a fantasy, not provably true, and hence probably not true.
In other words, a lie.
A statement based on a non-fact is a statement based on a lie.
And a decision based on (a statement based on) a lie, is always going to be a bad decision.
If only because there's nowhere to fall back.