Not impossible, but highly unlikely
The technology described does exist. Scientifically this is called Plant Microbial Fuel Cell. However Bioo's claims of 40 W/m² seem greatly exaggerated.
People involved in e-Plant, a Dutch company which already has similar product on the market have published a number of scientific papers in Biotechnology for Biofuels journal. Most relevant one is "The flat-plate plant-microbial fuel cell: The effect of a new design on internal resistances", which talks about improvements in efficiency, which are still nowhere near 40 W/m².
With the flat-plate design current and power density per geometric planting area were increased (from 0.15 A/m2 to 1.6 A/m2 and from 0.22 W/m2 to and 0.44 W/m2)as were current and power output per volume (from 7.5 A/m3 to 122 A/m3 and from 1.3 W/m3 to 5.8 W/m3)
As for question if they plant in pot would be able to charge cell phone, for example iPhone 6+ has 11.1Wh battery, charging losses on Li-Po batteries can be neglected.
Giving a Bioo very generous benefit of doubt, and assuming that they did something revolutionary to bring that to do 20 W/m3, such a flower pot might generate 100-200mW. Which would take 120-60 hours to charge iPhone. So it's not really feasible even with generous assumptions in their favor.