This answer looks like spam to me, but 2 weeks of vacation might have impacted my judgement. I want your feedback since flagging as spam is considered heavy handed.
No, I wouldn't consider it spam. When an answer like this involves code in some way, it may more-likely be considered self-promotion, if even that. I do agree that it's hardly a code review and is mostly saying how it can be done a different way, but it's still not quite spam. The answer can still be downvoted and commented to tell the answerer that an actual code review is preferred.
As for what could make it look more like spam (at least if it were duplicated on multiple questions as well), it would probably be shorter and just say how great the linked code is for solving this task. But even then, it would be mostly self-promotion than actual spam. To be on the safe side, I would only consider an answer spam if it has nothing to do with code, and instead mentions some entirely unrelated product or service (or usually some other nonsense). That's the ideal definition of spam on SE.
In this case I believe it is not spam. It suggests an alternate algorithm (using a stack, and regex), and provides a link (with disclaimer) to an alternate library. The answer satisfies the minimum requirements, I believe.
Whether this makes for a good answer is independent of that though, but, spam it is not...