A biodegradable HDPE? How could it be? If it is HDPE then it must be durable and not biodegradable.
Fine! Technology is on a fast constant change. Scientists must have mixed something in it, making natural degradation possible.
HDPE or the high density polyethylene could be biodegradable if added with special degradable additive during the manufacturing process. The same is true with PET.
The substance could be the oxo-biodegradable additives which requires sunlight to initiate the process. Biodegradation time depends on amount of additive added and length and intensity of sunlight exposure. It still hurt the mother earth if placed in landfills, canals and other waterway systems.
On the bright side, biodegradable HDPE can be combusted without the fear of adding to greenhouse effect. They have zero carbon footprint thus never emit carbon dioxide to air. Hope this is true!
It can be reuse as is but not recycled into other forms. It still need a responsible disposal scheme, else, have the same pollutant effect.