Linking to alternatives is not exactly constructive criticism for someone looking for feedback from a Show HN. It feels dismissive and out of spirit of what's trying to be accomplished here. Without any commentary or questions, it's almost like you're saying, "Why did you bother?"

Now, I very much doubt that was your intention, but please have some empathy for the person taking a chance when sharing something on Show HN. It's not easy and I'd hate for someone to give up on something just because someone linked to other things that already existed in the world. We wouldn't have Dropbox or Google under those conditions.

At the very least ask the creator how his project is different from these other projects or ask if he's seen these alternatives or even ask why he might have decided to build his own. A little bit of effort on your end shows you recognize his effort and isn't tone deaf to what's going on here.