Closures can capture values from their environment in three ways
You need this context to see the text isn't describing the traits or all values but individual(or groups.)
If any single one is captured the "once" way then the closure can only be called once. If none are captured the "once" way it is still possible to be called only once.
The big problem I see with the text is it mixes up with the traits and should define "the closure’s environment" outside of the once way. Not made very clear is for once there are two moves.
Think I just need to type once one more time.