A compostable product should not be assumed to compost properly in a landfill.
Landfills generally do not provide the controlled temperature, oxygen, moisture, and microbial conditions used in composting systems. Conditions can vary significantly, and the intended composting pathway may not exist there.
This is one reason disposal instructions matter.
A compostable bag is most useful when it supports an actual organics collection and composting system. If the bag is used for mixed trash and sent to landfill, its certified compostability may provide little practical end-of-life benefit.
The product, waste stream, local collection rules, and processing facility all have to work together.