> You are talking about ambiguous wording in contact language, which is very different than it being ambiguous whether someone was entering into a contract or not. The latter is not a thing
Of course it's a thing? There are loads of cases that turn on the question of whether a contract has been formed (and specifically whether an offer has been accepted).
> both parties have to reasonably understand they are entering into a contract for it to be valid.
To be honest, my initial impression was that the only reasonable interpretation of a thumbs up emoji sent in response to a text asking to confirm contract terms is that it is an acceptance of the terms. I see that you and others disagree, which is fair enough - maybe there is room for a different interpretation. Then it falls on the court to decide what the intentions of the parties actually were, on the balance of probabilities. Here, the court (having access to more background information than either of us) has concluded that the parties probably intended there to be a contract.
> It can be implicit acceptance (such as paying a cash for something without first signing anything), but the action that constitutes entering the contract must be clearly-defined.
The action in this case is very well-defined - it is the sending of a thumbs up emoji. The question is what was intended by it. There is nothing uniquely ambiguous about emojis - law libraries are full of cases where the court has to interpret ambiguous actions, or written or spoken words. There is not, and never has been, a rule that communication in a medium that is capable of ambiguity cannot be the basis of a valid contract.