Wow, this happened to me in dynamic queue

That's not anything to do with dynamic queue. The matchmaking system has been off for a pretty long time, something which Riot is working to fix currently.

is actually common for upper silver to get plats in dynamic.

Oh huh I didn't even know you could check other people's ranks like that, I'm gonna start doing that especially with my ranked games

And yea, matchmaking is kinda...not as good as it could be