You can't really make money by commenting on Reddit, they are very agressive about affiliate links and banning
Maybe it's true that Reddit's moderation is sufficient to prevent this from happening, but I feel like I see a decent amount of SEO-adjacent subreddits already, for things like new crypto pump-and-dumps. Before Reddit removed nsfw subs from r/all, you'd sometimes see a bunch of spam links for OnlyFans sites.
I could imagine a situation where Brave has to try to distinguish good subreddits from bad subreddits and it becomes just as challenging as distinguishing good websites from bad websites.
EDIT: If previously the race was to make a business look legitimate (page rank), then now it is about making users look legitimate (human rank)