Typically this special sentence/rule is used in scenarios where OL units have a specific area/item to reach.
I.e. it's their primary objective and they only attack a Hero if they have the opportunity to do so.
The unit takes the path that allows it to reach its objective (or come closer to). Hindering comes after ("if possible").
Let's say a OL unit with 3 base mvt points has to reach the "closest chest".
- One chest is 3 areas away, the other 2 areas away with a hero in-between. The unit can reach both with its 3 movement points, but it will choose the second because it's the "closest".
- One chest is 3 areas away, the other 2 areas away with two heroes in-between. The unit can only reach the first, so it will choose it even though it's not the "closest".
- One chest is 4 areas away, the other 2 areas away with two heroes in-between. The unit can reach neither, so it will choose the second because it's the "closest".
The unit's primary objective depends on movement. So you consider hindering only for this. Not for a potential opportunity attack that may happen after the movement.
THEN, when the unit has moved (and possibly completed its objective), IF there's a Hero in the same area, it makes an opportunity attack (affected by hindering if it only has a ranged attack).
Note: it can reasonably be house-ruled that IF the OL unit ends its movement in an area without Heroes:
- If it has only a melee attack AND there's a Hero in an occupied adjacent area, it attacks that Hero.
- If it has only a ranged attack, it makes a ranged attack against the strongest Hero in LoS.