Because the author doesn't want to intimidate new users by making them feel this is necessary knowledge to learn react. As someone presumably more knowledgeable in the field, it's much easier to judge whether an article is at the right level for you than someone just starting out.

I really don't understand your issue here.

A beginner might be put off using React because they don't understand a blog post about it written by a core contributor. Dan recognised that and added a warning to help those people not be dissuaded. That's basic, friendly writing.

Your complaint about it makes it look like you believe the post was written for you personally, with no regard for the wider audience. That's idiotic.

The disclaimer might serve an important purpose: reduce the amount of questions the author receives via email/Twitter. The disclaimer is obviously not written for you who can decide for yourself, but for people who are quick to write questions instead of reading the damn thing.

How do I know? Because I have disclaimers here and there that are obvious for most people and still help to reduce the support burden.

I am not a “beginner” and I appreciate the disclaimer. I value my time: i know the level of the article and i decide to read it when i have more time.

I don't understand why a beginner would skip an article like this. I understand as a beginner I may not understand what's going on. Shielding myself from knowledge I might not fully understand slows down learning. Messy learning by trying to absorb more than can be taken in yet is more effective because the mind will store the parts not understood and you'll be able to connect it as you learn other concepts.