I also trust them when they say it's not a simple matter. Implementing dark mode probably requires them to restructure and hire more developers. But that's the whole point, right? To achieve complex things to stay ahead of your competitors like OneNote and Keep. I'm an amateur when it comes to coding so maybe I don't understand, but amateurs on the internet have already build a semi-stable prototype of Evernote with a dark mode that works. If amateurs can do it in their free time Evernote surely could do it with the help of professional developers, a full access to the proprietary software, lots of moneys, and some elbow grease.
You are clearly a person that I admire immensely, a person that helps stupid users like me in forums so I don't want to bicker but you must know as well that when users mention "lazy programmers" or "fake news" their feedback is hinting at something missing, in this case, a dark mode! And as a person that stares at a computer screen 12 hours a day dark mode is one lame fad that I can get behind 😎