It comes down to how you interpret the counter charge rules, especially the highlighted part.
1.12.4 Counter Charges
An assault represents a brutal short-range battle involving movement, shooting and close combat. Although the assaulting formation will have initiated the combat, the defending formation will have time to react to the enemy assault and make limited moves of their own. To represent this, units from the defending formation involved in the assault are allowed to make a special move called a counter charge.
Defending units that are not already in base contact with an enemy unit are allowed to counter charge. Units with a speed of 30cm or more may make a counter charge move of 10cm. Units with a speed of 25cm or less may make a counter charge move of 5cm. Counter charges happen after the engaging formation has finished moving and any overwatch shots have been taken, but before the combat is resolved. All the normal charge move rules apply, and defending formations must still be in a legal formation after the counter charge moves have been made (ie, all units must be within 5cm of another unit from their formation). Embarked units may dismount
For me having a unit within 15cm after movement has finished is part of the "normal charge rules" so count for counter charges too.