Klaus made that comment right after reading his son's report on the king/priest's plan that made him /facepalm
And the misunderstanding get solved right before the human invasion, so it's not that long in the future. (well, quite some chapters to go in term of manga)
Sadly right after Ruri made up her mind to tell them she's actually human but before she told them, an IDIOT went and blurt it out, making things rather awkward.
Btw, the Dragon King became quite smitten with cat-Ruri, because a lot of dragons like small fluffy things, but being dragons they tend to scare them away, so Ruri not being afraid of him make him quite elated.
Later on he did get a peek at human Ruri too, and he was like "oh, THAT's why I suddenly like this cat so much and also lost interests in finding that unknown girl I met earlier"
After which point his doting became even more doting.
For example:
Ruri "I can eat by myself"
Dragon King *keep smiling and refuse to not feed her himself*