I guess you're right. I checked out the substack domain on HN and only found one obvious Show HN thread for it, and the overall list of threads was surprisingly short. I feel like I've seen enough Substack threads come up here that, if Medium were linked to for a lot of Show HNs, that Substack should be on that list somewhere even if it's dead last. My impression that Substack links were competing with Medium links on HN was definitely not correct.