It isn’t exactly. They created a list of known extensions by their id and a file which is known to exist in that extension. The site iterates over each pair and tries to load that file, if it doesn’t error it knows the extension is installed. It’s a clever and difficult manual process, but it does bypass the security trying to prevent this kind of thing.

I read that their reasoning is it exists to block users that use known scraper extensions which bypass their terms of use. But don’t entirely buy that.