According to the Oxford Dictionary (Google), the definition of biodegradability is as follows:
(of a substance or object) capable of being decomposed by bacteria or other living organisms and thereby avoiding pollution.
The definition for the term composting is:
make (vegetable matter) into compost.
The definition for the term compost is:
decayed organic material used as a fertilizer for growing plants.
The defintion for the term decomposing is:
(of organic matter) in the process of decaying.
The general assumption is that "PLA can decompose in nature". However, it's no secret that PLA is only degradable under particular industry conditions, presumable using bacteria or other living organisms. Otherwise, it wouldn't even meet the definition biodegradable. The terms composting, compost, and decomposing do not apply to PLA, despite many speaking and writing about it.
At the very least, either the general understanding of the term "biodegradable" is wrong, or the term biodegradable is misleading, perhaps even greenwashing.