That's still 25GB/month of bandwidth they save. Who knows, maybe that nudges them out of a high-cost data tier.
If he reworked old code just to save 50 bytes, that would probably be a mistake, but it sounds like the work was being done anyway and he had the choice to save 50 bytes OR use a longer domain name.
Basically this. I thought it was an interesting side benefit that came at no additional cost to the main benefit of greatly reducing request overhead for static resources.