Considering that both chains are supposed to exist at the same time, and having forked apart from a common block, how would the longer chain have a lower difficulty level if both chains adhered the difficulty retargeting protocol?
I've answered this above:
Usually when we reorganize one or two blocks, then the blocks will obviously have the same difficulty and therefore represent the same amount of work, so the longest chain will be the chain with the most work. However, if a fork lasted long enough to significantly stretch beyond a difficulty retargeting and in to a new difficulty epoch, then blocks on each chain would represent a different amount of work and so the longest chain may not necessarily be the chain with the most work. Nodes will switch to a shorter chain if that chain has more accumulated work.
Usually, the longest chain does indeed have the most work, since as you say, when the chains fork each block on each chain adds the exact same amount of work. If the fork continues past a retargeting, then the chains will have different difficulty adjustments since they will not have found all the blocks between the fork and the retargeting in the exact same amount of time. From that point on, the blocks added to each chain do not add the same amount of work, and so the longer chain will not necessarily be the chain with the most work.