Some projects help you with this by featuring a quick rundown of similar software on their website/repository readme along with links to the repositories. I always appreciate that.
Someone else's Show HN should not be an avenue for marketing your project, plain and simple. Maybe people will start organically talking/comparing to a competitor, but even that is different from the ol "hey everyone, here's my alternative!" which is just sheepish.
Time to leave the theater so other people can enjoy the show.
Let's say there's a dominant contender in this space that always blows any Show HN out of the water. So what? That should be a different post. If we're going to see ourselves as a group of hackers, then we should be able to talk about each other's standalone work. It's just become super corny when these threads are used as marketing for other projects.