Hate when they do that.

Dang. I didn't even know that.

EDITED FOR CORRECTIONS

From what I can gather, it's specifically this:

"Also included are previously unreleased bonuses – the Tinkerbat Transformation, Blue Shantae Costume, and Classic Risky Boots costume!"

These three things were originally only for Half-Genie Hero backers.

Additionally, it also comes with a new outfit for Shantae, a blue bikini with sunglasses.