That was nowhere in any of your definitions.
A legal fiction is a fact assumed or created by courts which is then used in order to help reach a decision or to apply a legal rule.
Legal fiction - Wikipedia
fictions are creations that are assumed true in order to reach a specific legal outcome. They are used by the courts yes, that does not mean they cannot be created or enshrined by the legislature.
I guess a legislature can write and pass any group of words, but I don't know of any example where a legislature created an "assumption or supposition of law that something which is or may be false is true, or that a state of facts exists--which has never really taken place."