Protective coatings are becoming used more and more extensively in many fields, and they can produce astonishing results.
That said, with applications for sugar-based structures, the slightest flaw, scratch, cut, or perforation becomes a barn door through which bacteria can drive their metaphorical horses and carriage.
But for temporary applications such as packaging, a sugar-based material with a thin coating (maybe plastic, maybe another material) to preserve it for a short life, but allow it to biodegrade or compost could be excellent. For example, I find it particularly ironic that when the garden is out of season, I go to the grocery store only to find "organic triple washed" greens packaged in permanent plastic clamshells that will last for years. Sure, it does the job of getting the greens to us, but then it is making the problem worse.
What I was thinking of is similar to wax coated paper cups.
See firmicutes [1] and bacteroidetes: cellulomonas, thermobifida, clostridium. Bacteria also experienced those hundreds of millions of years. In fact, there's some cellulose melting bacteria in your gut right now! ;)
But, you are right that it's not just bacteria [2].
To keep conveniences like my iced caramel frappalattechino we need a fast biodegradable, but like 3 hours degradable not 3 min degradable.
No idea if this is that, but “artificial shapes made of sugars like cellulose but not quite cellulose” sounds like you might be able to design something good that isn’t a forever chemical.
Remember SunChips? They made a “this multi-layer plastic snack bag can actually biodegrade AND keep our snack grease from leaking out on the shelf” chip bag. But they killed it because every crinkle sounded like a gun so people hated it. We still haven’t figured out those kinds of materials that work well.