I think what they were referring to specifically was Drew and Hunter double-penetrating a woman together. Even though this Brenna scene is non-canon, I think the information revealed during it is true, meaning Nora/Drew/Hunter threesome happened in canon.
Regarding possibly drugging Keira and Nora, at this point we don't know if drugs were involved back when they were getting fucked by Drew and Hunter. I like to think Drew didn't need the drugs, but may have used them due to Hunter's influence.
I'm dying to know how Keira and Nora feel about their past; they refused to discuss it with David which implies some kind of shame, and they definitely look unsettled in the teasers as you mentioned. I imagine they had years of therapy to try to get over it, and may have repressed their sexual natures (which Sterling will have to help them with lol).