They're confusing a few things by saying kanji is written top-to-bottom. Direction applies to all Japanese text, not just kanji -- which are simply one type of moji (characters). The traditional vertical format is called 縦書き (tategaki), and unlike the modern horizontal style (left-to-right), it runs top-to-bottom, right-to-left.
Here's some tategaki lyrics I wrote as an example:
https://codepen.io/kingcharlesone/pen/abgdPgZ
And if you want a really crazy (worst case) localization test, here's tategaki which includes English, Arabic, Hindi and some math:
To be nitpicky, there's also 縦中横(tate-chu-yoko, "vertical mid horizontal")/横向文字(Héngxiàng wénzì "horizontal direction literal character") in which [0-9a-Z]{2,8} are placed horizontally in the middle of a line.
1: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/P...
This is an excellent point, which of course complicates things further :D I'm going to update my worst-case Unicode now.