Considering Hokusai was Japanese, the descriptions are probably written in Kanji. The word “kanji” itself means Chinese character, so I wonder what exactly is your point?
"Kanji" is often translated into English as "Chinese character", so the two terms tend to be used interchangeably even when someone is referring to text written in Japanese.
In English, it would be most appropriate to call Chinese language characters hanzi, and Japanese language characters kanji, but this is an English distinction, not one in the native tongues that use these characters.
As the author of these drawings is Japanese, the correct term would be “kanji”, but the word “kanji” literally means “Chinese character”.