You're right in part, but standards aren't binary, they involve messy interpretation. There's no law of the excluded middle—it's all middle and it's all included. So instead of asking 'what are the axioms to be universally applied on this site', you need to ask 'what in this context will make the site better or worse in the long run'. I know that's unsatisfying but we can't do better.
So the point is more like this: had we a way to allow only the replies to job ads which are true and fair, while filtering out the misleading or disgruntled ones, that might make the job board better overall. But we have no such way, and the damage caused by false or misleading replies is the bigger risk.
Job hunting and hiring both suck, but there are limits to what we can do about it here. Allowing Who Is Hiring threads to turn into a combination job-board-plus-Glassdoor-style pelting range would not benefit the community in the long run. Knowing HN's limits and respecting them is one of the secrets of keeping this place going well. Not that it's a secret.