If I spoke with you on the phone about the contract and sent you the contract later on WhatsApp with a message please confirm the contract. And you answered with thumbs up. I would assume that you are confirming the contract and not that you received it.
That’s fine. We would both interpret it differently. A lot of reasonable people would. That’s my point. It’s not that a thumbs up can’t mean approval, but that it just as well can mean acknowledgment.