It is either solely or not solely. I chose my words carefully, and nowhere did I say luck isn't involved. Indeed, you've discovered rating doesn't always reflect how good a player is, though it can be an indicator of it. Some of the most respectable players in my eyes play without regard for rating and play to play. After all, you don't get much better by sitting on your rating. Have those players much better than you been above your rating? Why didn't they sit if they knew it was safe? Bad matchmaking absolutely does play a part in if you win or lose a game, but do not treat it as it is the only thing. It's something you can't do a whole lot about. The population is so small that you can kind of gauge who you just might get on your team too and who you may play against. Something interesting would happen if people were around the same skill level in your games. Matches would come down more to how each player performed in that game alone, which would limit your ability to climb too. You would also be playing against a team stacked with good players. You being the weak link would cripple you SO hard. Many that are diamond would never see it if feast were more populated, and if feast were more populated, better players would come naturally.
With your question, are you meaning to tell me that those players couldn't have done any better in the games they played? Did you watch them? Do you know what constitutes a mistake? I'm telling you that the single biggest factor in if you get something as simple as top 100 is YOU. I believe there's a reason we see the same faces at the top, and I think it's beyond foolish to think it's only because they are consistently lucky. Is there something you are wanting to argue? Why did you separate "bad matchmaking or lose streak in blind week"? So the losses were caused by something else other than bad matchmaking? In other words, the player in question? Did those players much better than you make it their goal to get top 100 all season? Did they do everything they could to make that goal a reality? I'll steal the answer to that question and say nope.
There are many things you can do that can give you a slight edge in feast that many do not do, yet these things pale in comparison to playing well—now imagine combining them. There is one thing that's impossible to do, and that is play perfectly every single game. Knowing this, it's all about limiting your mistakes, and that's what I think is a good definition for a good player: someone who makes the least amount of mistakes. I've noticed something throughout the years as well, and that is people can play a perfect game in their head and say they made no mistakes, while others will beg to differ and laugh at them. People might be making these same mistakes every single match, and they don't even know it, and if they don't know it, they will likely never change it. No one has ever challenged their play and made them understand why what they are doing is bad, and it can't really be fixed by practice makes perfect, but rather perfect practice makes perfect.