I think it may be a wording issue in the description. Instead of 'triple the drop rate', I think they meant the bushes/shrubs drop triple the amount of a single plant. But even by that definition, we should still get a minimum of 3 petals from every collection with the possibility of up to 6 (until we are maxed out of course). Considering that my single plants rarely drop 2 petals anyway, I'll be happy with the minimum 3 petals from the shrubs, due to the space and energy savings. (as long as it remains at least 3!)
I've never gotten more than 2 petals from a single flower either. So far, there is no boost or knowledge shard that increases flower drop rates. But that would be a nice addition to the game!
In this instance, Moon was referring to the promised triple drops from the new shrubs and bushes that we can craft in the arboretum.