Why would they think it's a great idea to claim they implemented CSS and JS from scratch when the first thing any programmer would do is to look at the code and immediately find out that they're just using libraries for all of that?! They can't be as dumb as thinking no one would've noticed?!

I guess the answer is that most people will see the claim, read a couple of comments about "how AI can now write browsers, and probably anything else" from people who are happy to take anything at face value if it supports their view (or business) and move on without seeing any of the later comotion. This happens all the time with the news. No one bothers to check later if claims were true, they may live their whole lives believing things that later got disproved.