I tested it with a cleric, who has it as one of their starting skills. It should be useable. If the problem was that you don't meet the requirement, then usually you simply can't use it at all. For example, make a Fighter and lower your Strength to 5 - the skills are greyed out and you can no longer use them. In Divine Light's case, it is not greyed out and you can cast it just fine, it does the animation - but it does not highlight anything in it's AoE and has no effect on anything there.
The Knight also has a starting skill which has a trait requirement, and yet they can cast it just fine from the start
So, no matter what's going on, I feel confident in calling that a bug.
In any case, trait requirements for skills are probably not sticking around, because it prevents companions from using them.
