Dark mode is now everywhere (this blog has it). It’s both accessibility-driven and aesthetically pleasing. But it often isn’t the highest benefit feature (and rarely, if ever, a feature that leads to product-market-fit). It’s not a technically difficult thing to build — across my entire blog, supporting dark mode takes up about 20 lines of code. For more complicated UIs, or retrofitting legacy applications, it’s a bit more involved (not to mention the technical debt of supporting it for future changes).
Dark mode is one of those 'not excited that you have it, but i'll be very upset if you don't' features. On 2x products I worked on that lacked it, the demand was tremendous, placing it in the top #5 demand. By itself, you're right, it would not lead to PMF but lack of dark mode created friction that spurned users to look for other items they did not like to justify decisions on whether to stay or leave. An unusual experience.
Whenever I see "now with dark mode!" on a product I'm already using, I roll my eyes.