Going that deep, I believe the black level difference stem from the fact that on OLED screen you can only see reflections from the surface, whilst on LCD you can see reflections from the surface, plus reflections from the bottom of the LCD coming back through the inner layers.
Much to my annoyance, 99% of anti-gloss screen covers are glossy. They are designed with the matt surface pointing towards the screen (or have it in the middle layers), so they do stop reflections from the back of the LCD, but do nothing about reflections from the surface. It's a very mediocre effect. By many such trial and error, I found this one
https://www.aliexpress.com/item/4000777080317.html really good, as it's sticky even the other way around with the glossy surface to the screen, rather than the matt. Since then, I always buy it when I need some smaller matt cover and trim it to the size myself. That way I have the best matt screen for Onyx Boox Max (Even the original Boox one was mediocre). By the way, in hindsight I regretted upgrading from the original non-touch Boox Max to the touch one. The screen resolution is way better, but the glossy touch surface was annoying. Luckily, I eventually solved the problem.
Interesting discussion anyway. I agree with you about the matt vs glossy surface, absolutely. My subjective observation was, however, that glossy OLED wins against glossy LCD in terms of reflections (thus comfort of use).