Interesting observation, some thoughts on how this might have come about below.
Due to its roots, Hacker News started out with a community heavily biased towards people with entrepreneurial tendencies who are often optimistic by nature. Over time more tech people were told about Hacker News and started participating. These people in turn brought in other people from other academic fields (e.g. law, medicine, engineering, physical sciences etc). Many of the newcomers, especially those who had training beyond their graduate degree, are likely to have been immersed in an academic culture where ideas are put up to be critiqued. At a high level the goal being for ideas to be tested in a collaborative fashion and any weaknesses addressed so they can be improved. Everyone working together to move their field forward through the refinement of shared knowledge and understanding.
Having spent some time doing post-graduate research in science then transferring to working as a self taught software developer in industry - this difference in culture is something I've become aware of multiple times.
In my experience executives and business development/sales oriented people often struggle dealing with technical colleagues who point out problems in their ideas/plans. They perceive the challenges as negativity or pessimism instead of an attempt to refine the idea/plan to something workable or less risky. On the critics side, they have been sufficiently interested in the idea/plan to give it serious consideration so it can be disheartening to be ignored or rebuked for offering a critical response with the aim of refining the approach.
I've also noticed this difference in approach even when working with other software developers who went into industry straight out of university. From my perspective many of them don't routinely apply critical thinking in their work. They rarely question why the system is designed a certain way and simply try to jam whatever feature they are implementing into the current design even if it is a bad fit. This usually means adding many changes in lots of places throughout the code base, because it is quicker to do by the deadline (pace over perfection). Compared to stepping back a reviewing the business model at a higher level which would take longer to change but make the code easier to maintain in the long term.
You noted pedantic comments get upvotes. People who are deep in the details of a field have to be particular about word and language use to avoid misunderstandings and make communication with other people in the field more efficient (this often leads to heavy use of jargon). Marketing and sale oriented people like to be looser with word use to portray something with the best possible interpretation (ie "organic" products).
As a simple example, in a previous job I refactored an old code base doing Monte Carlo, Markov Chain simulations from doing each iteration sequentially in a single thread, to being able to run in parallel in multiple threads. For many workloads this meant a significant (many hours) reduction in simulation time. For other workloads the added overhead of collating all the results back into a single data set meant a slight increase in run time (a few minutes). The marketing people wanted to claim an X times speed up when promoting the new version (best possible interpretation for increased appeal to customers). I wasn't comfortable with that as I knew some existing users wouldn't have that experience (being pedantic).
I would see the marketing teams version as being way too loose with language (bordering on being untrue at times). They in turn saw my revisions as not sexy enough or too wordy. It took effort from both sides to get to something we were all comfortable with.
Having said all the above, I agree that sometimes HN can be unnecessarily negative of people's fun projects/weekend hacks. I sometimes like to play with sub-optimal solutions to problems because I have a sudden inspiration to try something and don't want to spend hours researching perfect approaches. These projects usually don't have any commercial or academic purpose, it's just an itch being scratched and an opportunity to do some greenfields coding with no restrictions (not something I get much opportunity for in my current job). I have seen other people's projects torn to shreds when they happen to make it onto HN and it always makes me a bit sad.
Is the critique and pedantry good or bad? I'd say it depends on the situation. Sometimes very good information or ideas will come from the critiques and the pedantry can help eliminate ambiguity from a conversation and lead to some new and interesting places (one of the main reasons I come to HN is the high quality of these conversations). Sometimes it can devolve into academic point scoring for no apparent purpose.
Maybe something for people offering critical feedback (including myself) to consider would be to lead or end their contribution with a positive note. This would at least signal their intention is to collaborate on getting to something better.
I'm sure what I've said above will be picked apart. I have generalised a lot to keep the response shorter. I do not believe all startup founders are optimists or technical people are all pedants etc. My point is more that the HN audience is composed of a wider mixture of people from different fields than when it started. This undoubtedly will affect the nature of the conversations.
I hope this response doesn't come across as negative :) It has been interesting to me to think about and speculate as to why it has occurred.