As has been implied, the COI guideline is nuanced, and so the best advice
is to keep on the safe side. The terms of use of the site in respect of
paid editing are, on the other hand, clear cut. The former relates to
intention, the latter to factual matters that are easier to discuss.
I would
Whilst I agree and gave exactly that advice yesterday to someone who then wrote
a review of an article on a talkpage explaining where her publication
contradicted it, another editor promptly responded with the suggestion that of
course she didn't have a COI and she should amend the article
