If a company sells apples and they start pushing social media campaigns about why oranges are bad and they think you should be buying apples instead, then that’s a conflict of interest.

I think some people are missing the conflict of interest because the author blurs the lines between his personal opinion and his business. He turned his personal brand into his business and uses his name as his brand.

EDIT: I think people are missing the meaning of "conflict of interest". Having a conflict of interest doesn't mean something is wrong. You can be right and have a conflict of interest.

The issue is that conflicts of interest exist whenever someone's paycheck depends on an opinion being true. If someone on Twitter was alternating between trying to sell you vitamin supplements and then posting articles about why those vitamin supplements are good for you, HN would have no problem with pointing out the conflict of interest and taking it into consideration in the context of evaluating the claims.

If someone's entire job and personal brand are built around selling courses for particular stacks, that's important context to bring up when they start writing about why other technologies are bad. Agree or disagree with the conclusion, but you have to acknowledge that the writings should be read with the conflict of interest taken into account.