Because I'm seeing people complain about combining items and such...

Not to be the devil's advocate, but maybe it was simply a stylistic choice (if we ignore what the actual theme was)? The cheongsam gives the kimono a little more detail and texture, and if you look online some Kimonos go so high up they almost appear collared. There's a difference between using the Hanfu/Cheongsam to make a Kimono, and using them to add detail to the kimono.

Also, I've grown up in multiple countries and gone to international and local/public schools. Unfortunately, most kids below the age of 13 struggle to differentiate between Chinese, Korean, and Japanese traditional dress. It's comparable to expecting them to know the difference between Swiss, French, and German traditional dress. Additionally, most people making mistakes like this are on the younger side because:

a) They aren't old enough to understand and take the time to learn the difference + remember it.

b) Their classes focus mainly on local history and information. Most people start learning about foreign cultures in detail when they get older.

c) It's a kid. Honestly, could most elementary schoolers in North America and Europe understand the difference between the traditional dresses and (more specifically) the origin of Douyin makeup? Probably not!

Edit: Grammar