I haven't made it to that quest yet, the ranger light armor message is a bug; the message occurs frequently when the armor in question is actually light, but the game doesn't realise it.
All armor, light, heavy or medium applies a penalty to skills like Hide, Move Silently and Tumble. The extent of the penalty depends on the armor, light armor has a very small penalty, heavy armor a very large one.
What the message is trying to tell you is that you won't be able to use your ranger two-weapon fighting or archery feats in medium or heavy armor, but if you're sure that the armor you're equipping is light, then the message is in error.