Well cannabinoids are definatly neuroprotective in regards to MDMA induced neurotoxicity, but why that does it is unknown... it's probably not CB1 receptor mediated, but it could be antioxidative of via reuptake inhibition...
the experiment in question isn't the best, so it's hard to comment.
Compton-L337, lets just go through some things point by point
1) Cannabis increases the amount of released serotonin in the brain
2) Cannabis increases the amount of released dopamine in the brain
3) There are study which shows that delta9-THC inhibits the uptake of serotonin (at quite high levels).
When a chemical blocks the uptake of neurotransmitter, it means that there will be more of that neurotransmitter floating around.
It's likely, however, that the increased serotonin (and dopamine) is caused by THC, activating the CB1 receptor on GABA releasing neurons, and reducing the amount of GABA released. GABA is an inhibitory neurotransmitter, so if there is less of it, there is less inhibition of neurons which released serotonin and dopamine. Hence, more serotonin and dopamine.