In Vray and similar renderers you can specify a factor for GI send and GI receive in Override materials. I guess that's what they're referring to, and probably GI receive factor of the plant materials was increased, which would make the leafs pick up more GI energy and appear brighter. In Corona, you can override GI send only by using a RaySwitchMtl.
I guess it's not really possible to do the same in Corona. You could increase GI on the ground's material but that would also affect rocks, buildings and anything else that receives any GI from the ground. Another thing to try would be to use SSS/fraction on the plants so they sort of 'accumulate' more light.