You do realize that the internet has this thing called hyperlinks that you can use to send people to another website? Just in case, let me show you: https://twitter.com
Boom! You were on HN, now if you click that link you're on twitter! Magic!
Boom! You were on HN, now if you click that link you're on twitter! Magic!
Sending people away from your content on the social media site to view it somewhere else results in less interactions with the social media content -- there's not 20 tweets all of which can be replied to, liked, retweeted, etc, there's just one tweet (and a blog post that the social media site can't see).
Thus, giving people less things to interact with means that you'll receive less rewards. This is the "punishment".
It's "punished" in the sense that there's a relative negative consequence -- your post is raised up less by the algorithm than competing posts. This is colloquially valid.