At some point in the recent (last few years) past, reddit account bans became a fully-automated process with no oversight. Unfortunately, that leads to people getting banned for nothing. That happened to me.
In this case, you did violate reddiquette. Historically, you were expected to make 10 posts in a subreddit for every one self promo post. But violation of this usually led to a subreddit ban, not to a reddit ban. Also, no one actually cares about reddiquette anymore, not even Reddit.
I expect that they auto-ban self promos these days because they want people to pay for ads.
In this case, you did violate reddiquette. Historically, you were expected to make 10 posts in a subreddit for every one self promo post. But violation of this usually led to a subreddit ban, not to a reddit ban. Also, no one actually cares about reddiquette anymore, not even Reddit.
I expect that they auto-ban self promos these days because they want people to pay for ads.