User blog comment:B214/Question and Answers 2/@comment-86.81.216.93-20140529173038/@comment-1359322-20140529234345

Also Yu-Long never resigned as Dragon King, he is still one. The only member of the Dragon King that relinquished his position is Tannin because he became a Devil.

For starters all 5 of the Dragon Kings were already inactive before the start of series, Midgardsormr is sleeping, Vritra is sealed, Fafnir made a contract with Azazel to be sealed, Yu-Long and Tiamat were hiding. And all this guys are still considered Dragon Kings.