I flagged this question as spam. The flag was declined. This was surprising to me, hence the following question.
What is the definition of spam being used on MSE?
The definition of spam as should be adhered to in the flags is:
A post should be marked as spam ONLY when it contains an unsolicited advertisement.
Jeff Atwood also spoke rather directly about this:
And by "spam" I mean it in the strict traditional internet definition not as shorthand for "I don't like this post."
Linking to several previous questions is not an unsolicited advertisement. Also, because the effects of "helpful" spam and offensive flags are more serious than those for regular flags, the moderators are perhaps more meticulous about deeming them helpful.