Just finished the game and tried my best searching for the collectables. It said 100% feathers on the save screen, yes, but does that mean true completion?

Here are my progress in detail: I collected all pink feathers, and got a total of 30 green feathers, number for each chapter: 3, 5, 3, 5, 1, 7, 5, 1.

I've also found every secret level, except I can't find one in Derelicts.

I've found an easter egg at the very first Chapter where I went back to the left after triggering the claw, then climbed up in one screen.