I think he is saying that there is an airdrop component in the merits gained by some senior members when they increased rank. I was a Hero when the system was introduced, so I received 500 merits, and thus, I only needed an additional 500 "earned" merit to gain Legendary status.

No one ranked up through airdropped merits because they already received the respective merits equivalent to their rank. For example, if you're a full member before the merit system implementation, you'll receive 100 airdropped merits. So it means if they achieved the rank, it's through earning merits since the introduction of the merit system. I'm one of those people who are close to being a Sr. Member and the merit system serve as the barricade for me to rank up.