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 approach the topic from the direction of paid editing, making the
point however that COI need not arise from a financial interest.
Charles