This is intentional. You have to use your arrow keys, WASD or HJKL to navigate.

However, it's impossible to read longer comments this way. This comment https://news.ycombinator.com/item?id=28705415 in this very thread is too long to fit in my browser window, which is maximized on my 16" laptop. So far I'm just pressing O, but it still takes time. A scroll for long comments would be really welcomed.

Other than that, it's a very cool UI! I can see using it full-time and press O only when I want to reply. Actually, I just did :D

You can actually press spacebar to open a comment inside a modal where you can scroll.

Wow, thank you very much! :D

Mixed with keyboard tabbing, it gets really confusing.

This is true. I recommend to not use keyboard tabbing while navigating throught the comments. Makes it a lot less confusing and you still do everything.