"Fuß fassen" essentially means "to become established" or "to settle down" depending on the context. For instance, you could "Fuß fassen" in a new country, or a business could do the same in a new market. The example sentence is a bit vague without more context but I'd read it as, "Even just the fact that it's been [able to become] established as an anecdote shows there's some truth to it after all."