In this particular instance, I don't think the algorithms tag is appropriate. The reason here is simple: the author didn't ask about an algorithm. Sure, the question is probably one arising in the analysis of algorithms, but that doesn't change the nature of the question. Let me provide an example from another field.
Say that a well-meaning student asks, on the physics site, how one might go about converting a set of differential equations into an equivalent set of integral equations. These happen to be Maxwell's equations and, naturally, everybody recognizes them as such. Should the question be retagged to indicate the relationship to electricity and magnetism?
In light of this example, I have another suggestion entirely: not only was the retagging inappropriate, but the question might not even be a question about computer science. Indeed, this - and the example involving Maxwell's equations - are questions about mathematics. The plot thickens.
I am not necessarily suggesting that questions involving the use of mathematics be removed from this site; that would be a disaster. Certainly many people here are qualified to answer questions like that, and in a sense, a lot of the content we can ask (and have been asking) here is a kind of mathematics. I believe that a good solution might be to introduce a mathematical-methods tag, or similar, for situations such as this. Questions about solving equations, proof techniques, etc. which would find application in solving computer science problems could be assigned this tag.
For what it's worth, I'm not even sure that algorithm-analysis would be a fair tag for that question; the author doesn't even use the word algorithm. It's just a question about working with Landau notation.