LEO isn't the problem. Anything in the orbit these are in has a lifetime of a few years at most unless they have active propulsion (i.e. not debris or microsatellites).
The problem with space debris is mostly higher orbits where it will remain in orbit indefinitely. In LEO, the atmosphere does a good job of "cleaning up" debris and inactive satellites.
This launch was to 500km, and the Doves are around 150kg/m^2 (4.5kg / 0.03m^2). This puts their lifetime at around 5 years according to that linked plot.
For reference, Planet's earlier launches were to the ISS (400km) and lasted less than a year, which your plot agrees with.
The problem with space debris is mostly higher orbits where it will remain in orbit indefinitely. In LEO, the atmosphere does a good job of "cleaning up" debris and inactive satellites.