The principle is that you may not copy without permission of the copyright holder. When the copyright holder says "Feel free to use this for commercial use", that is very clear permission and only requires that you know English. Saying "License: CC-BY-SA-NC" is not clear, because it requires you to know what the code "CC-BY-SA-NC" means. It's not impossible to figure it out, but in fact it is just a magic expression that most people do not understand, and in this case, the creator probably relied on someone's say-so and did not understand what "NC" means. Clarity always trumps obscurity in the law.
This assumes, of course, that the person posting the item is in fact the copyright holder, so is entitled to say "License: CC-BY-SA-NC". There is a separate and huge problem that there is virtually no way of knowing whether a person who posts material has that right. You can be pretty sure that anything owned by Disney is not posted with permission on a non-Disney site, but for most material, you just don't know. Someone falsely implying they have the right to give permission is a higher level of evil.