Do you need to grind the plastic into dust for that to happen?

No, AFAICT.

Everything goes faster in hot compost with additional preprocessing, but it's not strictly necessary to reduce things to dust.

Hydrolysis of the PLA still happens whether it is dust or something larger. After that, the chains are small enough to be available for microbes to eat fairly quickly.

They're just eating one bite at a time; they don't know that they have an entire elephant to eat.

More surface area improves immediate availability and speed, but less surface area doesn't cause the process to cease.