How does a fuse affect creepage reqirements?
No.

IEC 60079 Qualified rating will mean it can be used in an explosive gas environment (even if you will not do it, the label will say it can be done), this means that any applied or induced voltage cannot be allowed to spark. As such at any point, fused or unfused, applied or generated, the creepage distance is fixed for any given voltage. No if, and or but.

If there is even a milimeter of copper not as specified and that is registered during examination or testing of the device, it will not be approved for explosive gas or military norms until changes are made and a full re-examination is done.

Your presentation is on a whole other IEC norm, that for medical equipment, so you cannot simply compare those and assume "oh well, for the one is for the other". The forum post is for standard IPC guidelines, again, something entirely different. If your IEC 60079 prescribes a creepage distance (and I think 4mm is low for 230VAC, so do re-read the section where you saw that) you need to stick to that everywhere. 

And be aware that norms like those also lay strict specifications on PCB material grade and stack-up limitations. At no point, in no way, can any charge cause any polarisation of gasses strong enough to form a point of ignition. None.