Indeed, the manufacturing is more difficult because the silicon has to be thinned after fabrication of the circuitry. The lucky feature of image sensors is that the image is actually shifted to the edges of the chip by an analog shift register mechanism. But they still need an electrode structure to make the shift register work, and the light has to pass through it in front-illuminated sensors.

Any indication if this backlit image sensor development was inspired by nature (the octopus here, for example) or was just an independent evolution of the technology?

Independent. Nobody would do Frontside illumination if it weren't a lot cheaper and easier because you very clearly lose out on light.