My goodness, the generation of (good) answer posts is what Stack Exchange is all about!
I would say, and I'd expect that most would agree that taking information left in comments and using it to generate an answer to a question is excellent form and is exactly what the commenter had in mind when leaving the information in a comment.
By all means, go for it!
update: Just spotted in Science Fiction SE, not exactly the same question but same sentiment in answering: What is the proper etiquette for answering old questions?
By all means, answer it! Unanswered questions are a blight that needs to be fixed!
(To more be more specific: if a question hasn't been closed, then old questions definitely need love. There are even badges specifically for providing good answers to old questions.) (emphasis added)