I2C actually sends inverted data over the line?
You should not take that diagram as literal as you do. It shows how the electrical guys typically implement the hardware interface, and which explicit and implicit resistors play a part in shaping the waveforms. 

The I2C standard states how a bit it transferred over I2C, that is the final word on it. It states that a 1 is transmitted as high on the SDA line. The 'sending FET' must be high-impedance to make the line high, hence we must feed it the inverted signal on what is called 'SDA out' in the diagram (in order to comply with the I2C standard).

If the author of that diagram had cared about signal polarity, he would have called it *SDA_out (the * means inverted) or the equivalent in another convention.