Models are charged at the model level.
"must attempt to move into base contact with as many opposing models in the enemy unit as possible with as many of their models as possible"
Units declare charges and declare target Units to charge.
But it's the models that charge other models.
The initial charger moves into contact with the closest enemy model. After the first model is moved, other charging models can be moved. If possible into base contact with an enemy model.
The section ends with;
"all the models in the charging unit <snip> having engaged as many enemy models as possible with as many charging models as possible."
It is totally possible to Charge an enemy Unit, where only a single Model is in Reach.
Why is there a difference about *how* the enemy models are out of Reach? Does being out of Charge distance differ from being a Swooping FMC? Both are out of Reach of charging models.