Reddit was able to scale by handing off moderation to the communities themselves and to the unpaid volunteers who wanted to moderate them. In general, I think it is obvious to any casual observer that those volunteers don't see moderation in the same way (or with the same goals) as the platform. For example, many (most?) moderators on Reddit absolutely do ban people not because they are starting flame wars or spamming but because said users aren't toeing the party line. A huge number of subreddits are created specifically for that purpose – "this community has X opinion about Y and if you don't like that you can GTFO".
However even if you ignore the unpaid volunteers moderating subreddits and focus only on the "Admins" that were specifically chosen by Reddit, you can see that the only priority was not increasing the signal-to-noise ratio, including during yishan's tenure. In most cases when a community is banned it is not because the signal-to-noise ratio is too high but because that community has received too much of the negative PR in the press that yishan referred to. Sure, the claim is still "we're trying to maintain the integrity of the platform as a whole and are banning communities for brigading, etc", but you can see based on which communities are banned that this is clearly not the whole story.