Unranked to Diamond in 7 hours

[Iyrnwaen]

This is entirely possible because they implemented a Streak Bonus mechanic. So they could see that a player is streaking and purposefully try and throw them into a harder match.

You don't think it's a bit silly to suggest they implemented a streak bonus to make it possible to rank up with a win rate under 50%,
and then undermined that very system by artificially torpedoing your matches if you get a streak? Wouldn't you just... like... not implement the streak bonus in the first place?

Even speaking from my own experience I felt like I had the exact situation happen to me. I won 5 straight matches getting to Silver 1 and on my promotion match into Gold they put me against Gold+Platinum players, opponents and teammates, even though I was still Silver technically.

I ended up losing, and from that ended up losing my next 5 out of 6 matches. The whole time leading up to that I've never seen Platinum players during Silver, because it should be Bronze - Silver - Gold.

This is how you would expect a functional matchmaker to work.
If you're playing at your skill level, you should expect a win rate of very close to 50%. An even match should be a coin toss.
Let's say for simplicity that you're at 1k MMR and you go up or down 100 MMR depending on whether you win or lose.
If you were to perfectly alternate wins and losses, something that's statistically virtually impossible, your MMR would effectively never change.
Instead, you get something like your example, where you win 5 matches in a row and your MMR skyrockets.
It's now a full 50% higher than it was 5 matches ago, and the matchmaker is appropriately going to start putting you in matches with players in that MMR band.
Either you'll win those matches too and continue to climb until you reach another plateau where your MMR stabilizes,
or you'll lose until your MMR falls back to its previous equilibrium- and while it's unlikely for those losses to all come in an uninterrupted streak, it's certainly possible.