The phrase is used all the time, even by the UN.
And it is not true - even if you believe that Israel is occupying territory and that it is not 'disputed.'
International law recognizes occupation as a descriptive state. Sometimes it is called 'belligerent occupation.' But by definition, any occupation is by default legal. The acts that an occupying power perform can be illegal under Geneva, but the legality of the occupation itself is not addressed in either the Geneva Conventions or the 1907 Hague Conventions, the only two sources of international law for occupation."