Why don't you message the creator (Nate Baldwin[1][2]), and bring it up?
...though since the creator referenced not only the Wikipedia article, but several others on that site, perhaps it's not a typo?
---------
[0] https://en.wikipedia.org/wiki/Helmholtz%E2%80%93Kohlrausch_e...
[1] https://colorandcontrast.com/#/references
[2] https://natebaldw.in