I always thought it was the opposite. That subreddits allowed the community to expand without catering to the lowest common denominator, and reddit would have fallen off the cliff faster without those.
Similarly, I think that Reddit optimizes for groupthink even more than Twitter does. Good luck having an insightful conversation in /r/pol or /r/conservative or even some technology subreddit.
And mind, I think Twitter Inc is absolutely awful and Twitter is a societal ill. I just don't perceive a significant degree of difference between it and Reddit with respect to discourse.