Every time somebody participates in a community, they take on a role in that community's story and that role has a certain shape.
Many people behind the posts here on HN are far less serious than what's suggested by their posts because the community sees humor as too easy a way to derail curiosity and intellectualized dialog. And people are generally more contrarian here than off-site because the community celebrates a good and interesting debate as a way to raise and explore questions. etc etc
Is a HN user being artificial when they choose not to be snarky? Are they being artificial when they pick into an argument they'd have shrugged off elsewhere.
It's no different at LinkedIn. It's tone of positivity may not suit you, but it's not really a different kind of behavior than you see here or on Facebook or Reddit or whatever.