The content here isn’t fake, so I don’t think this strongly resembles generative AI. As far as I know it’s also legal to astroturf in this way because there’s no claim that every poster is a real user. Even today there are Reddit bots although by convention they make themselves obvious.
IANAL but if one were to claim in public that the fake users were real (and there were actual damages) then that may well be actionable in a court of law.
I don't think this (comments by someone for commercial reasons not being indicated as such) is legal in the UK, I can't cite the legislation on it. Pretty sure it was in the raft of business requirements that the EU bundled, one of which was business being required to provide a trading address.
Even if you did claim every user was real, how would that be illegal? Defrauding investors is illegal. But fake users isn’t necessarily defrauding investors.
IANAL but if one were to claim in public that the fake users were real (and there were actual damages) then that may well be actionable in a court of law.