Of note: It's A Summer King, not THE Summer King. There are 3 Queens of each Court, so why not as many of more Kings? It seems more like a courtesy title, something won by might instead of directly related to the Seasonal Courts.
EDIT: at 7:12 in the youtube video of the DragonCon interview he says specifically "The Erlking, he is a Summer King."
And Santa Claus is "one of the Winter Kings" - plural
Sorry, in almost all of the quotes in my compilation I was diligent to take the notes word for word, but that was one of the most recent quotes for the earlier iteration of the compilation and I was running out of words so I had to edit down.
Having forgotten that I was pretty much forced to edit that one down some, I was assuming that the quote in the resource was verbatim which it wasn't.
Really the whole point of my responding was to counter this:
yes, it stands for WoJ, and seems to be mis-quoted quite often. 
(especially when it comes to the Erlking, who everyone assumes Jim says is a King of Summer, but *NEVER* actually says that. 
And you pointed out that not only was it implied, but he did actually say "the Erlking is a summer king"
As to the nature of the titles. He does say that the kings are related to the
Seasons. The Kings courts however are separate from the queen's courts. So far when the winter court is referred it is usually in context to the winter
Queen's court, but remember that Mauve's court has a level of separation from Mab's and it seems that the Respective seasonal King's courts also have a much higher degree of separation from the queen's courts.
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