This protein has been previously shown to be a proofreading enzyme in RNA viruses – that is to say every time the genetic code of the virus replicates, nsp14 scans through the newly-created genetic material to make sure no errors have cropped up. Mutations in proofreading enzymes spell disaster for organisms that don’t replicate often, so in a virus (which enters the cell, replicates into thousands of virions, and bursts from the host cell in around 10 hours), a faulty enzyme would spell utter catastrophe.
These mutations appear to contribute to a crippled virus that is unable to replicate, which could explain how the Delta variant simply vanished from Japan in a matter of months.
The theory is certainly interesting but doesn’t quite explain why the crippled virus would outcompete the dominant strain