The electromagnetic field that causes "interference" is generated by hardware in the proximity of the soundcard. The whining or buzzing sound of capacitors or the VRM on the GPUs could very well be picked up and transmitted to the headset, but only if the resulting electromagenetic field is oscilating at an amplitude and frequency that matches the natural resonance of your soundcard hardware. To safeguard against this, they put shielded components on the sound card board, which drowns and absorbs the EMF. It doesn't matter if the signal is optical, it will be transmitted as part of the audio waveform.
Having said that, I've had various add-in sound cards in my life, which I've installed next to GPUs, TV cards and the odd network adapter and I've never encountered interference as a result.