You can argue that Washington's words did not amount to a "sellout" because they were a more practical approach to black rights than more radical stances would have been.
There is a saying that we should not allow the "perfect to be the enemy of the good." This applies to Washington's situation. In the South of the late 1800s, it would have been perfect if blacks had been given full political and social rights and had been treated with complete equality. However, this was simply not going to happen. Therefore, it made sense for Washington to propose a "good" solution where a perfect one was not possible.
Washington's solution would have been somewhat practical. It would have kept blacks in a subordinate position that whites would have accepted. At the same time, however, it would have encouraged whites to help blacks develop economically. It might have even led to greater respect for blacks. In this way, the proposal can be seen as a practical accommodation to reality, not as a sellout.