Why though? That is what likes are for. Likes mean you just like the post or you agree with it. And overall depending on context everyone will know why you liked the post.
That is your interpretation, but that is what they are for. We don’t need to clutter the interface. You liked something about it. What does “agree!/same!” say that likes don’t? If you agree with something or can relate to it you would use likes anyway.
First of all, this may not be implemented by the forum itself. Try suggesting to Discourse, the software this forum uses.
Also, why would a new feature be implemented if it’s going to be used rarely? The like button can be used for any reason. You could also DM that person.