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That's strange. By including the link you satisfy the "BY" obligation of the license; but what about the "SA" one? Unless your code was being released under an open source license.

As parent said, this is not legal advice -- ask someone qualified to answer.

I wonder how this applies on proprietary code? Should the attribution be added in documentation or third party notices?

Out of curiosity, how does the audit work? Do they have automated tools to check against open source code bases and SO snippets or is it manual? Interesting nonetheless...

Yes, there are a number of tools that analyze source code and compare it with "all known source code on the Internet". They usually produce list of matches. For less-than-complete-file matches, usually human review and decision is needed.