I agree. The main benefit of stating a piece has been posted elsewhere is to avoid being accused of plagiarism. Your content is yours to crosspost/syndicate where you choose to.
The term 'self-plagiarism' is irrelevant here on a blogging platform. It's relevant in academia and research where using your previous content/arguments without clarity that you're recycling content is considered to be a serious breach of trust.
Attempting to apply the word here would mean that anytime a person puts a post up that has any language previously used in one of their posts without referencing the post they would be self-plagiarizing. That would apply to all those posts that have repeated parts along with updated information. That gets pretty silly, pretty quickly in a blogging situation.