Tl;dr: you are in effectively the hole (but stricter) for anything between 1 day and months, without charges. It is torture. As in actual torture.

Fact check... anyone can confirm this treatment is standard in Japan?

You can try looking at this manga posted by a Japanese person that was detained for marijuana possession https://xcancel.com/kime_neko/status/1634511023167381504. It's in Japanese, but you can use a machine translator and/or look at the drawings.

The facilities and food look slightly better (maybe because it's a detention centre in Tokyo), but it mostly matches. Although the mangaka seemed to have a much more positive outlook on it, probably because they could read all the Japanese books they wanted and speak to their cellmates in Japanese.

Pretty standard.