At the moment, the New York Islanders can't sign Sorokin for the 2019-20 season. The NHL is still holding the arbitrary line that, as NHL Deputy Commissioner Bill Daily stated: "Each team has kind of set its roster" (subscription required).
So until the NHL and NHLPA decide that teams can sign unsigned draft picks to current year deals, as it states in the CBA (see Critical Dates Calendar on page 344), Sorokin is likely going to mull all of his options.
One of those options was to possibly stay in Russia for the year.