The writing brush is a product of China, so everything in it is Chinese characters, just like clothes, which were copied from Japan and South Korea. The ancient books in Japan and South Korea are mainly Chinese characters, so their calligraphy is also Chinese characters. But Japanese and Korean calligraphy is different from domestic calligraphy.
Just like Yan Zhenqing's calligraphy is different from other running scripts and cursive scripts, it's just different in style.