A typical GPIO works at 150MHz without risk for EMI but why is there a risk of EMI at even 400KHz in I2C?
You need much faster slew rate than 50ns to achieve 150 MHz square wave, which means much larger frequencies and much larger EMI.
The point is not the signal frequency itself, but how fast transitions it has. I2C is meant to just work without worrying about impedance matching, so it is also designed not to need it, so anyone can wire it and it will most likely just work, because it is supposed to have slow edges. A 150 MHz clock is so much harder to get right regarding even PCB trace impedance and source termination.
Sometimes GPIO pins have so strong drivers that it will generate ringing on the signal, no matter if it is only toggled at 1 Hz. Of course since it happens less often it is not usually a problem regarding EMI, but signal integrity.