I do think it was always there for Diana, they just had to embrace it.
Did not know that about Dick. I do think he's the easiest suspect out of the four if they made any of the bi/gay/pan, especially since people talk about it the most in regards to him. And, it's something I would love to see explored but I definitely don't think they'd give us two queer Robins.
Personally, I do see it for Tim so it was more of a "good for him" moment, not confusion when I saw the story. But I imagine it's hard to either make either Dick or Tim bi when their relationships with women are so high profile. (Tim and Steph to a lesser extent but she was brought up when he came out).