Forum does not matter
Whether the material are Youtube comments visible to everyone with internet, or an internal report of a company prepared by one person for the CEO’s eyes only, copyright applies exactly the same way.
There can be non-copyright issues, but I assume those do not apply here, for instance:
- classified information (e.g. nuclear bomb blueprints - but see U.S. v. Progressive)
- privacy laws (e.g. photos of a famous actor in his private house)
Journalists publish stuff, news at 11
In the united-states, in france, and I assume in every functioning democracy, journalists publish conversations, notes, documents etc. without the consent of whoever produced those documents. The point of investigative journalism is that they publish stuff that other people would rather keep hidden, without getting sued to oblivion for it. (Arguably, countries where this is not possible tend to devolve into non-democracies at a relatively quick rate.)
Journalists are not protected in themselves; rather, activities typically associated by journalism are. In the US, that is part of the doctrine of "fair use". (Pedantic note: "fair use" is a US-specific term, and its use as a general term for similar copyright exceptions in non-US jurisdictions should be avoided, because the exact scope of the exception varies a lot across jurisdictions.)
What’s the line of fair use?
In the united-states, fair use analysis famously uses a four-point balancing test.
In the case of journalism activities, #1 (purpose and character of use) will always weigh very heavily in favor of the defendant.
In most cases, #4 (impact on market value) will be favorable as well. Here, few people can be expected to pay for a book "the full comments under Youtube video #4256231", and not just because they are public.
Finally, #3 (amount and substantiality) might be more or less unfavorable. Citing the full conversation makes a fair use claim weaker on its face, but one might argue that the full conversation is needed to understand context of the good bits. The conversation might also be as short as a few paragraphs, or fill a full book length.
The balancing test leaves quite a lot of margin of appreciation to the judge. However, I believe the case presented would almost certainly be fair use. Cases that clearly fail #3 and #4 (for instance, publishing in extenso internal reports of a company) have been judged fair use before.