I think the problem isn't dark mode itself, but as the author states "native dark mode support". If your app is designed in white-mode, it's important to know how it is going to look in dark mode, and native support somewhat limits your control over that. This is particularly egregious in email.

This is less of a problem in apps and the web, but in email it is absolutely horrendous. But then again, the entire email styling thing is still in the dark ages.