That might be true in principle, but even if we counted Lenin and Co as a bona fide pupils of Marx, they hardly exhaust the class of all Marx pupils. So even if we discount Lenin, there are plenty of other pupils left.
I am inclined to believe that Lenin and Co were NOT Marxist in any true sense - they were Russian modernizers who selectively used Marx as building block of their managerial ideology (i.e. a set of myths justifying the powers and policies of top executive officers in an organization) - other elements being peasant populism and Russian nationalism.
While their economic and social policies were the best thing that happened to Russia and Eastern Europe throughout the entire history of her miserable existence - these policies were "Marxist" in the name only. They lacked two key elements of Marxist thought: (i) the idea that certain historical developments must take place before socialism can be implemented and (ii) the idea that exploitation occurs not through nominal ownership but through the appropriation of labor time that such ownership entails (i.e. workers are exploited not because they do not own a factory, but because capitalist has the means of making them work longer than it is necessary to reproduce the value of their own labor power) - while for Marx freedom from exploitation was to be accomplished through shortening of the work time, the Soviet state was notorious for elongating rather than shortening of the work time.
Wojtek