According to the 26th sunken scroll (human skeleton with Wii-U) in the first Splatoon's single player campaign the games take place about 12000 years after 2015.
So yeah the Inklings evolution is quite a bit to fast for the theory of evolution's usual time frame requirements.
Also given the Octarian's affinity for flying saucers you could probably "ancient aliens" your way into an acceptable evolution theory.

Would make sense for the lack of conventional biology, otherwise, bones would probably be rather rare instead of a common find for a child digging to splatoon's version of China.