Probably just biodegradable in industrial composting facilities unless you add HDPEase enzymes to it to break it down. But perhaps the filler material makes it less dense and easier to break apart, so easier to digest for bacteria, etc..

High-Density Polyethylene (HDPE) is not biodegradable. HDPE is a type of plastic that is known for its durability and resistance to degradation in natural environments. It is often used for items like plastic bottles, containers, and various other products because of its robust properties. However, these same properties that make HDPE plastic so useful also mean that it does not readily break down in the environment.

To clarify, biodegradable materials are substances that can be broken down by natural processes into non-toxic components, such as water, carbon dioxide, and biomass, with the help of microorganisms. HDPE does not meet this criteria, and it can persist in the environment for a very long time if not properly managed through recycling or disposal in a controlled manner.

If you're looking for more eco-friendly alternatives, you might want to consider materials like PLA (Polylactic Acid) or PHA (Polyhydroxyalkanoates), which are biodegradable plastics. However, it's important to note that even biodegradable plastics may not break down in all environments and may require specific conditions to degrade effectively.

Almost any organic compound is "biodegradable" given enough time, the right environment, and the right species. But the simple practical answer is no, HDPE is quite non-biodegradable. Researchers are working to identify ways to biodegrade polyethylene-based polymers, and it is possible to a degree (see this review), but naturally these polymers will take hundreds of years to break down. I also found this blog post by a packaging company that addresses the question.

Could you post a specific source of the claim? It may be that they have a more specific explanation of their claim somewhere on the site.

It is tricky to judge because different countries have different rules and standards for when you can apply the "biodegradable" label to a product. And merely claiming that the product is biodegradable without actually placing a label on the product can also be subject to different rules. So it is difficult to figure out what they mean without inspecting the source. Their claim could be based on something like 80% of the material being biodegraded within a specific timeframe, or they could be making a misleading claim based on being "technically" correct and making the claim within an under-regulated context. Or.. They could also be making a misleading and illegal claim, possibly not on purpose.