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.