Did they truly transgress adultery?
Rashi (from Sanhedrin 93a): Yes. Achav told [Nebuchadnetzar's daughter 1 ] 'Hashem said that you should consent to have Bi'ah with Tzidkiyah, and Tzidkiyah told her that Hashem told her to consent to Achav.
Presumably she was married, for it says "Neshei Re'eihem." The Gemara implies that she asked her father, and do consent! Perhaps the verse considers it as if they fulfilled their intent. (PF)
Why does it say "ha'Yode'a va'Ed"?
Rashi: "Va'Ed" is an expression of testimony.
Radak, Malbim: I know that they speak Sheker in My name, and I am a witness to their adultery.
Radak citing Targum Yonasan: It is revealed in front of Me, and My word is a witness.
