There are lurkers but many of the active commenters are frankly just more active in voting and commenting. Lemmy has much more engagement than Reddit for commenters because it’s a small community and the way comments are default sorted. You don’t need to make a comment early to end up on the top of the thread. You just need to get about two up votes and it’ll put you at the top. If what you said is worthwhile then you’ll get a lot more after that

Edit: one of the benefits of this is it encourages high quality late comments. On Reddit those would largely get ignored, but here engagement like that is encouraged.