In flex queue, why is it that diamonds can get matched against silvers?

I have had multiple streaks of Dias/Plats in my silver games. WAs terrible going from S1 to G5. It literally wasn't until I deranks to S3 that I got paired vs mostly Gold opponents. Then it slowly normalized.... until I got to promos and started get Dias/Plats again wtf.

Flex queue MMR is separate from your SoloQ MMR so Diamond players in SoloQ can potentially be matched with Silvers and Golds in FlexQ

They don't, show me a game where that happened and I'll explain what you don't understand.

It's flex MMR, has absolutely nothing to do with Ranked Solo/Duo. Its not taking account for any ranks other thank Flex Queue. It's completely balanced imo, because if it took in consideration Ranked Solo MMR then it would just be Ranked Solo with more than 1 duo.