Depending on dual taxations agreements of their country of origin, they most likely would have to pay tax if they followed the law. There's often a clause in the different dual taxations agreement that says that describe how to determine tax residency in case the person hasn't stayed in any country more than 183 days. Usually, it's the country where that person has the "centre of vital interests" or sometimes can be determined by nationality.

Digital nomads are still too new a phenomenon that countries really properly seek to tax them but just traveling like this without being careful or creating a company in a country with low taxation is not necessarily safe.