Grumman wasn't behind the times, there simply wasn't a call for folding wings in the BuAer specs. until the USN needed to increase the aircraft number aboard ship.
Add to that, typical folding wing configurations in use were all patented and required licensing.
Grumman's innovative "Sto-wing" design (introduced in '41) bypassed other designs and became an industry standard.
In the case of the IJN, very few of their aircraft had folding wings and the ones that did, only had folding wingtips in order to clear the elevator - the Aichi M6A being the exception.