I always see/think of 
as having the meaning you’ve attributed to 
(“I agree with what you said in this post.”/“What they said.”). Whereas for me, means the exact same as 
(“I like the fact that you posted this”).
I was thinking this could be used to acknowledge something as being very “Apple-y” or appreciative of the Mac (/iPhone/iPad/etc), or just generally reminiscent of the past or spirit of early computing. For instance, I would have given a six-colour Apple emoji to this post, had it been available:
I’m not as keen on the Apples with overlaid + or - as I see it as more of a feeling than a specific rating. But of course, this is your forum, so if that fits better with your vision, I’d prefer any six-colour Apple option to none!
Maybe it was originally forced, or maybe I the other groups I’m on are unusual, but for the past several years I have seen this commonly used as “Thank you!” (as well as “Yay!”). But I’d certainly be open to another emoji for “Thank you”. I think having something for this sentiment would be useful.