If they had disclosed their involvement in their profile at least I could give them the benefit of the doubt but in this case, like the other commenters, I assumed he had used the tool as a customer and had a positive experience, not that he was literally the founder of the group making the tool.
That’s not to say his opinion is not wanted, just that the potential bias should be made transparent.
There aren‘t any DMs on Hacker News, and if he didn’t post that comment I would have thought it was a disinterested commenter recommending something they had used, not somebody who works on the project. The cultural norm here is to disclose when you are recommending your own product and it’s not childish to point out when people fail to do that, it’s reinforcing that cultural norm.
