This was an ornament I printed roughly 6 years ago. Being a Christmas ornament it spent most of those 6 years stored in my roof space.
Being in Australia this would have been subject to average temperatures of 30 to 35 degrees c but also peaks across summer approaching 70 degrees c. Also in high humidity.
The PLA crumbles into tiny pieces at the softest touch.
I thought it was interesting that PLA would start to break down in these conditions.
After reading a bit, it seems that you are correct. Under natural conditions PLA wont actually turn into non toxic compounds at all. Biodegradable just means that it can be theoretically done. The conditions necessary for it are however only available in industrial, heated and controlled composting systems. Without those conditions it will break apart but it will stay bad for the environment on a chemical level.
Just look up “PLA biodegradable test” on a search engine or youtube and you will find plenty of people that tested this.
PLA is considered non-toxic by itself already.
And while the biodegradability/compostability is indeed rather circumstantial, the much more important part is that it’s a renewable, plant based plastic. Currently the most useful way to get rid of it is to incinerate it for energy, which ends up being rather carbon neutral as it just releases the carbon the plant material used for growing itself.