In this question I want to reply to the second part of the answer which is spoilered (as I spoilered my question). But the spoiler markdown doesn't work in comments (>!).
Would it be possible to allow spoilers in comments?
Comments are intentionally second-class citizens and lightweight in terms of implementation requirements and user experience impact. I don't think adding another feature to them will be well-received. If “spoiler” comments existed at all, the whole comment would probably be a spoiler, and even then I don't think the feature has much chance of being accepted.
I don't like it much myself. Comments are already displayed in a low-key way; I don't think you'll be reading them by accident. I'd be quite comfortable with a “comments are always spoily” policy, especially on answers.
If you really need to include a spoiler in a comment, and you are not commenting on a post that implicitly spoils the respective plot-element, just prefix it with a written spoiler warning indicating what exactly will be spoiled by the comment.
[WARNING: Integral plot details of "Star Wars: The Empire Strikes Back" will be spoiled by reading this.] The argument in your answer is wrong, because when Luke meets Vader in Cloud City it is disclosed that Vader is actually Luke's father. So, he couldn't possibly have been a time travelling robot from the future.