Knowing how to actually build things from scratch in a way that survives the beating of time is something else. It’s much harder to quantify though. Also, these days you have the css in js solutions, which means you can build the app by dropping in the css where you need it, so you get to tick that box, but if you tried to do it the “traditional” way you might not even know where to start.
EDIT two of my more common phrases at work are “you could just use css for that” and “you don’t need to use flex box to do that”. I’ve found that there’s a lot more using css to style things in isolation without understanding the power and performance it brings to the table. I spend more time trying to stop react renders from happening because css should have just been used than I do on the micro detail of ordering things a certain way to avoid repaints.
As others have mentioned, getting people to assess themselves accurately is very difficult. We always seem to skew towards the overly-confident side - not least because this what is usually rewarded in the job market.
Although it might be worth considering a simpler heuristic in the future as the distinction between Advanced and Expert is somewhat vague and perhaps ultimately not very informative.