Are you supposed to drain the raisins before adding them? I'm guessing that the small amount of liquid here may make it a moot point, but I wasn'...

You're right...the amount is small, so probably moot.
That said, there might not be ANY liquid left. So, no problem.
If there is SOME left and you want a drier cake, drain the liquid (but don't throw it out - use it in some other dish, like yogurt or fruit salad), then proceed with the raisins in the cake recipe.