For advertising your target isn't necessarily someone organically scrolling who happens upon it. It's definitely the people who are googling "best cordless drill reddit" or digging through the stickied megathread of a specific interest's sub. In those cases it would be very very hard to beat the effectiveness of a well-placed comment.

Well places like r/mattresses are frequently astroturfed so I take that as at least weak proof casual readers read comments.

I think it depends on the subreddit. For instance many r/news readers likely only read the title.

But r/ama readers likely always read the comments because they are the core content.