popcorn and foamed styrene are both things that produce air pockets. And these slow down heat transfer.

popcorn is way less efficient than the stuff they use in refrigerators.
Further, the stuff wilts and compacts over time.
The vibration of the compressor in the refrigerator would speed that up.
It is not a good use case for popcorn.

However, whomever is assembling the refrigerator could use popcorn. There is nothing stopping them. And it may be essential if chemical foam isn't available.

This is one of the things about distributed manufacturing. Between one refrigerator and the next, there doesn't have to be any similarity.