Many people used to come here to shitpost before the merit system was implemented. Merits were airdropped for users back then; is this why they don't understand the value of being a legendary because they didn't earn it through their knowledge contributions to the forum, but rather by simply posting whatever they wanted?
What's the value of being Legendary exactly? It only shows you've probably made an effort to reach a particular rank. To acknowledge the contribution of a user, you just look up their earned merits. If most are airdropped, make your own conclusion.
Can we conclude that not all legendaries are knowledgeable in the forum?
Absolutely. Ranks don't grant knowledge. Earned merits (hopefully) do, and it depends on which sub-boards they're earned.