They explain it a bit here: https://mitchellh.com/writing/ghostty-and-useful-zig-patterns

Also, calling out the warning signs, my bar for a native platform experience is that the app feels and acts like a purpose-built native app. I don't think this bar is unreasonable. For example, I don't think it's unreasonable to say that Alacritty is kind of not native because new windows create new processes. Or that Kitty is kind of not native because tabs use a non-native widget. And so on (there are many more examples for each).

So nothing wrong with Kitty on MacOS e.g., but the "feel" is not native. Personally don't care too much about that, but the author seems to do.