To me, this tweet seemed much more anti startup trying to grow big than anything anti-YC. Only using the tweet for marketing as mentioned at the bottom.

At first, it's against dedicating your life to your startup (which presumably you are already doing or at least want to do) and then states that YC actually has a good success rate for unicorns.

Then he talks about doing some brainstorming to find ideas and product market fit, but at least from my perspective YC doesn't advocate for this at all, but rather talking to customers, getting their buy in and then making something that solves customer problems. Aka, not brainstorming at all.

And then goes on to say that YC has lots of young people rather than older (more likely to be successful founders). But doesn't do any of the data diligence on percent of founders by age by industry and by YC vs. not.

And finally he focuses on saying that only the unicorns were success stories and no one else, while advocating that you should aim for a smaller goal. But doesn't acknowledge any data of companies from YC that aren't unicorns but are still operating, sold for less than a billion, etc.

I personally think that looking from the outside that YC is not for everyone and is geared towards the VC route. But if you want those things, then these arguments aren't valid for why you wouldn't apply.