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?
Refereing to chinese character as Kanji is inaccurate. (Also the meaning of compound Hanzi and Kanji is often different, take "jiudian" as an example).
> Refereing to chinese character as Kanji is inaccurate.
"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 Japanese there is no distinction between kanji and hanzi, they are the same thing, and written equally - 漢字.
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”.
Not intentionally I don't think, but people might do that if they're not sure which language the characters came from or if they just don't know the term "hanzi".
But in this specific case, it is a Chinese character (because that’s what kanji is, and that’s what the word kanji means), and the character is actually kanji, since it was written by a Japanese author.