If playing a team one time it's not enough to rate a team, then playing 1 time 4 different teams it's not either.
That is such a basic logical fallacy that I'm not even sure where to begin explaining it to be honest. You're completely missing basics of statistics and also disregarding all the context that comes with seeing so many teams play so many games.
Let me take a more extreme example to hopefully help you understand. Lets say every team has played all their games this season except for C9 and TIP who have to play their final game. Lets say TIP has some record like 3-14 and C9 is 14-3.
/u/SinatraZ could then rank C9 some place much higher than TIP, while you could come along looking at his ranking and saying "how can you be "pretty sure" C9 will end above TIP after the huge stomp TIP gave them?" That would be ridiculous because your one game example means nothing when we have already seen overall that C9 is doing well against all teams.
In that case he would be taking in the context of all the matches played and where they are currently standing. As you can see having more information (especially at the end of the season) makes it easier to rank them, which is why his ranking that late in the season would be trivial. But that exaggerated case shows that having more information (i.e. more than one game) makes it easier for him to be accurate. That context is important.
That's not to say that 19 games played so far says everything about where teams are at, but it does show that having all those additional games (and not just looking at one) is relevant and helpful for predicting placement.
Liquid ended up first last split and then got shit on at playoffs and gauntlet. Was Liquid an average/bad team all along or they just had "bad luck" in each bo5 they played?
You're pointing to a single case and working backwards to design your logic around it. A prediction isn't a fact. No one is saying that it's guaranteed. The whole point is that he is basing his predictions off of more information than just one game, which you somehow think is equivalent.
And to use your argument against yourself, someone could have taken a single game from Team Liquid last season where they beat CLG or TSM and say "Do you really think either of those teams will be above Team Liquid after how they got crushed?" Of course not, that's ridiculous.
But I'm pretty sure you know that because I'm convinced you're trolling based on how trivial this logic is.