The key word is "relatively"... I did NOT say or mean to imply that local airflow across a wing is constant-speed.
Every chord section of a wing is moving through the air at the same speed when in stable flight, since it is in linear motion.
Outer chord sections of a propeller are always moving at significantly higher velocities through the air than the root sections, due to the propeller's rotational motion. In fact, tip speed is a limiting factor in propeller design.