They didn't say in what way the kid's handwriting is trash, but I've noticed that westerners who have spent time writing kanji end up with handwriting that looks like it was done by a machine.

Probably the same concept at play, but what helped me as a kid was getting one of those black Pentel 0.5mm engineering mechanical pencils (used to be $5 apiece or so at Staples) and using it to write everything as small as possible. On college-rule paper my lettering would occupy 25% of the vertical space while strictly adhering to the bottom line in neat rows.

Few teachers could read my writing without a magnifying glass, but they got their revenge by specifying essay lengths in number of pages. I had to do 75% more work to meet the page count.

I wasn't intentionally being an asshole with this-- in writing such tiny lettering it forced me to pay attention to clarity of my lettering to an autistic degree, otherwise my writing would be entirely illegible to the teacher and my work would be rejected.

Also of interest are Arabic, Hebrew and other RTL scripts. Not sure if more myth than fact, but their origin was apparently in stone masonry and right handed hammer use. I can't quite believe it though, given the difference of ease in sourcing materials and the curves present in the scripts.

I studied Chinese, and modern mainland Chinese is written left to right and a pain for lefties to pass Chinese exams with. But the IMEs are great so you never have to write them out yourself, so you just need to focus on how the word sounds and what it looks like.