Does no one recall the early party banter dialogue between Verric and Cassandra where Verric asks about Josephine and Leliana's relationship and Cassandra uncomfortably responds "They are.....friends" to which Verric gives a colorful "oh i see" type response? I've yet to hear even another bad explanation for that exchange. Could just be gossiping and totally unfounded assumptions i suppose.
The line from Cassandra you quote does exist - I give you that. But Varrick never asks about their relationship and there is no "oh I see" type of response. Though with lots of bad will one could interpret Cassandra's answer like you do, I give you that.
The dialogue in question, behold, the Scribbles-Birdy romance exposed:
Varric: You know, Seeker, for someone with your tact and charisma, you assembled a pretty good little Inquisition.
Varric: I'm giving you the benefit of the doubt and assuming you didn't drag them all here by force.
Cassandra: How kind of you.
Varric: I mean, you never know. You could have kidnapped Ruffles, and she'd be too polite to say anything.
Cassandra: Leliana recruited Josephine. They're... friends.
Varric: So there's a rational explanation after all. Just when I thought you had layers.