Oh, close. I do think Yokkyun is a jab at that “moe” Hinoue/Toei art style, but the direct reference is in the song that Keima sings for the insert of the Yokkyun focused episode. It’s called Happy End, and the lyrics of it quickly devolve into a mashup of Bishoujoge references.
The line in particular is:
Fate with you 暗い夜など
And it shows up at 1:19 in the song.
There’s also a couple references, one to Key directly, and one to… ONE. (Key to Heart 輝く季節へ)