I think the instructions to add the cubes after the fermenting phase is because fermenting wine will be warmer than wine that has been
through the primary fermentation. If you add oak
during active fermentation the increase in temperature could
cause you to get too much oak essence in a hurry. I think that is why it is suggested to add oak after you have racked off of the gross lees. Also, at that time you won't have to deal with strange smells of fermentation or thick lees as you try to decide when you have added the right amount of oak.
I have also heard that wine that has a high ABV absorbs oak essence faster than those that are not. The reason is that water is not as good a solvent as alcohol. Therefore, you can quickly over-oak higher
alcohol wines. More frequent test tastes are necessary.