Agree in that while verbally is fully a contract it is hard to verify unless recorded and possibly witnessed. Point being it is as legal as a written which many people do not understand.

As for the thumb up emoji, this particular person has used it in past and had a history of using it to approve services. In that context I think it is fully legitimate to hold him to it. Barring that, yes I agree on most cases it will not suffice as a legal document. It is much the same way a China often uses English for legal contracts as their written language has too many interpretations.