Looks like your horizontal platform needs another side attached.
If you have two supports on each end then I assume they will be separated by a 2" open space.
If that is the case, and when you refer to stress as how much weight it will support and it will be used as a bar. Then it will be strong enough to support a fair amount of weight provided you add the other side piece to the horizontal. You should be able to stand on this setup without a problem.
I'm not sure the L brackets help any more than resting the top on a 2x6 attached to the vertical planters. A couple more bolts on each end will help.