Right before the paywall in this very article:

Ukraine’s constitution is confusing. Article 103 states that the president is elected for a five-year term; but Article 108 says that he or she exercises power until a new president is inaugurated. A longstanding law (though not a constitutional provision) says that elections cannot be held when martial law is in force, as it has been in Ukraine since Russia began its full-scale invasion in February 2022.

Having read about this before, I can say that The Economist is wrong. It is a constitutional provision in Ukraine.