Earlier quoted context omitted.
I read a thumbs up in the same way I read "ok" which is affirming agreement. Me: "Want to goto the movies?" You: "Ok". We're going to the movies. I don't interpret this as you are acknowledging receipt of my message and will decide later whether or not you will attend. I would apply the same rationale to a thumbs up. You are saying, ok. Others may disagree.
This has tripped me up in stores. You bring 1 item and they tell you, "This thing is two for X (price)". If you then say "Ok" disinterestedly, will they sell you one or two of those?
Merely saying "This is two for x." is a statement, not a question.