A contract is a meeting of minds, with agreement between the parties.
Even a verbal agreement is legally binding. Of course, it is hard to prove such an agreement exists to others, so we create a record of the agreement.
If there is evidence that such agreement exists, then the contract is enforceable, and that is all that has happened here. Using a thumbs up emoji to indicate agreement may look a bit silly, but the intent is apparently clear.
Even a verbal agreement is legally binding. Of course, it is hard to prove such an agreement exists to others, so we create a record of the agreement.
If there is evidence that such agreement exists, then the contract is enforceable, and that is all that has happened here. Using a thumbs up emoji to indicate agreement may look a bit silly, but the intent is apparently clear.