The ranking is a joke. Look at taobao, the most famous website in China. Its front page is a mess. As an Chinese I can tell you those Chinese copycat software or websites are often patched up Frankensteins.
What does UI/UX have to do with HackerRank? I don't think HackerRank even measures more "pragmatic" qualities of software (e.g., code organization, clarity, etc). In any case, the "busy UI" or cluttered look that is almost universal across Chinese and east Asian websites and apps is often intentional because the chaotic UX is preferred by consumers. It reflects the experience of shopping in a Chinese market. It packs lots of assuring information into the page. It has historical and technical precedent, and breaking the trend would mean "sticking out". Sleek, simple designs may even be viewed with suspicion. Another factor is that Chinese text will generally appear more chaotic to Westerners who cannot read Chinese and are accustomed to simpler Latin characters and Western typographic conventions.
That only tells you, that they lack frontend skills/manpower. But if it's most famous, means it works.
Also, that tells you just about nothing about the site backend, where the coding happens. Programming competitions are not bootstrap time trials :)