A rain barrel is supposed to collect water to be used. If it has not rained for several days, then presumably the owner will take some or all the water to water plants.
When withdrawing the water if you use your bucket to siphon off the top water in the barrel, any mosquito eggs or larvae will be taken into the bucket and dumped on the plants. When the barrel is quite low dump it on a plant.
Secondly a rain barrel is likely to be under a rain gutter down spout. If it rains frequently the barrel will overflow and any mosquito eggs and larvae will go with the excess. All that paraphernalia in the article will just get in the way of removing the water.
-- Dale Creasey, Fair Oaks