The author is one of the creators of Tailwind CSS, which gives you a predefined color palate but no components, and is intentionally opposed to the Bootstrap ideas. The recognizable part of Bootstrap isn't the fact that errors are red, it's the shape of the buttons, inputs, etc., and the fact that all errors are the exact same red as other Bootstrap sites.
At the end of the day "good" UI doesn't have a lot of room to be different -- it actually needs to be recognizable -- but you don't need to change much to avoid being generic. Using different shade(s) of red for errors is one way.
The irony is that you can now often recognize sites being designed with Tailwind the exact same way. They tend to use a certain font, they tend to slightly overuse the card metaphor with a rather specific corner radius and box shadow...
When looking at this site I immediately though: aha, this guy has been using TailwindCSS to design the website... (until I realized that in fact yes, it was designed with Tailwind and the author is also the author of tailwind itself.)
So I guess it's the same as bootstrap at the end of the day. Both provide you with options to customize them. Most people don't.
That being said: Working with tailwind is amazing!
At the end of the day "good" UI doesn't have a lot of room to be different -- it actually needs to be recognizable -- but you don't need to change much to avoid being generic. Using different shade(s) of red for errors is one way.