What's the point of flex que? I still get matched with/agains plat-dia players

Yesterday I decided to play flex ranked so I could try the new kayle rework and try some weird builds with her, for my surprise I was against a riven otp Diamond 4, I didn't care about that, I can handle it, but why my team is below gold 1 facing this riven?? They also had a platinum player how is this balanced, I'm literally silver 1 in flex ranked