Many legendary members do not receive more merits than lower rank members.
If we turn the table around, I can say, "the same way many lower rank members do not receive more merits than the legendary members".
Atleast, I know may legendary accounts that get far more merit than any lower rank account - and with this, I will like you to know that earning merits has absolutely nothing to do with what rank a user is, but it has all to do with how knowledgeable a user is.
Legendaries that earn more merits earn it because they are giving out something useful to the forum, and likewise, lower ranks that earn more merits earn it for the same reason as the legendaries - knowledge.
Can we conclude that not all legendaries are knowledgeable in the forum?
A "yes" with a straight face.
The rank "Legendary", to the lower ranks on this forum represent "knowledge", if you permit me to liken this forum to a university, I did say that the legendaries are the professors.
But unfortunately, the rank "Legendary" in this current time does not truly represent "knowledge", because like NeuroticFish said in his/her post you quoted, not all legendary members active now attained that rank through merits, some of them were already legendary members before the merit system was introduced, and like a king that is already sitting on the throne can't be easily dethroned or removed, they were allow to keep the rank, but the merits only went to those legendary members that truely deserve the rank due to their level of knowledge.
So to put it clear, I would say that one of the ways to differenciate or know true legendary members is to look at their post history and merit score and history, this is the easiest way to know those who truely deserve the rank and those who are keeping the rank for old user sake.