Saki isn't saying that. What Saki is actually saying is that she's BEEN in love with Kanon so she's been watching her more than anyone, and Kanon had always distinctly referred to her as a friend and had never really looked at her romantically, even though her actions were really suspicious.
Which is true, Kanon herself didn't realize she was romantically attracted to Saki until a few chapters ago, so Saki is actually correct in saying this. To her this development IS out of nowhere, it's her first time seeing Kanon act this way. Heck, Kanon hadn't shown that she's aware gay people even exist until right now in Saki's POV. You can see Saki actively digging though her memories and seeing situations in which Kanon had distinctly "friendzoned" her or expressed interest in the opposite sex (like the actor from that one show).
Of course I will add that this is definitely a lot of denial on Saki's part, or at least the fact that THAT'S her first reaction is. Her logic makes sense is all. Although it's funny that none of the moments she seemed to remember were more recent, because Kanon had been growing more and more obvious (to the point where others and even Kanon looking herself could tell). I think it is telling that Saki chose to ignore more recent memories/actions in her recollection.