Earlier quoted context omitted.
The event handling alone is almost a hundred calls deep. Because a lot of the work is happening asynchronously, you won't see most of it when stepping through the debugger starting from a click handler for example, but try adding a breakpoint to the compiled JSX. With fibers (React >16) and a couple commonly used hooks you'll easily hit a thousand high call stack.
Do you mean that the async chains of something().then(somethingElse()).then(...), into which async/await code desugars, grow 1000 levels deep? I never encountered it, but, OTOH, I did not research this in particular. V8 very definitely does not produce a call stack out of it, but schedules every new Promise as a separate task. (A bit like a Lisp threading macro.) So, what forms 1000 levels of nested calls? Is that an…
The async comment was to point out that if you attach a breakpoint to your `onclick` handler, you will reach 'the end' of execution after less than a hundred function calls. But the actual work (see above) inside react and react-dom hasn't even started, as it's pushed to a queue. This may give the impression that far less code is running than actually is.
This is still in context of "you can read through the library's codebase and understand what it's doing fairly easily"; so yes, it's specific to React being very complex vs something like htmx, which most devs could understand in its entirety in one afternoon.