How is when it first flew relevant to how unstable it is?

Because it is an example of a totally unstable plane which flew three years before the plane under discussion.

The X-29 is an even more totally unstable plane.

Ah, I see. You want to argue that since the F-117's instability did not prevent its becoming a success, despite using fly-by-wire computers that were 3 years older (and consequently more primitive) than the X-29's, then instability cannot be the root cause of the failure of the X-29.

The person you replied to was expressing skepticism that the claim that the X-29 was "the most aerodynamically unstable aircraft ever built", on the grounds that the F-117 first flew in 1981 (which implicitly predated the X-29 which first flew three years later in 1984), and was unstable on all three axes.

You asked of what relevance the date first flight was, and I made the relation of first flights explicit, which answers your question of what relevance it is. That is all. Your claim about what I want to argue exists only in your own mind.