The image says it all really - I flagged when the question was still open so how come the question was then put on hold but my flag rejected?
The post was flagged as 'spam'. I did not think it rose to the level of spam as I understand that word, and so I declined the flag. (If a vendor creates a link to his or her site for a commercial product, particularly one which is irrelevant to MathOverflow, then that I consider to be unequivocally spam, and as such I usually destroy the account that created it.)
Meta stackexchange discusses the notion of spam here: What are the “spam” and “rude or abusive” (offensive) flags, and how do they work? (I would slightly broaden their definition, for the purposes of MO, to include cases where for example a crackpot links to his "research" on the Riemann Hypothesis; I'm pretty sure I have a sizable community backing on that interpretation.)
I saw no evidence that this post, which is bringing to attention a new volume of Bourbaki, was an unsolicited advertisement coming from the publisher. It seemed roughly on the order of someone creating a link to a vendor for a mathematics text, in order to point to literature or satisfy a reference request. Also, broadly speaking, a new volume of Bourbaki would be relevant to the interests of the MO community. Hopefully this explains the reasoning behind declining the flag marking the post as 'spam'. (In another sense the flag could be considered "helpful", as I was glad to have learned about the post and the community reaction to it, but following the menu options available to me, I thought my response was logical.)
I did not interpret the community reaction of closure as agreeing with the characterization as spam; it was closed for other reasons. Yours was the only spam flag raised.