I'd simply change the mains lead. You've no idea what damage has occurred inside. I regard IEC leads as expendable items and I've got loads of spares. Get a new lead. For peace of mind, clean the contacts in the PSU to reveal shiny metal. If you're tempted to use a file,
don't leave any metal dust behind. Clean thoroughly.
The two shorter "prongs" in the PSU (the engineering term is 'pins' or 'contacts') are for Line and Neutral. If they are badly pitted and blackened due to arcing, they need attention. I've seen far worse damage inside cheap mains on/off switches, where half the metal contacts have vanished (vapourised).
The guy on Reddit is correct. The ground "prong" should never arc, unless the PSU has gone badly wrong, the Line input shorts out to the metal case and a fault current flows down to earth. This fault current is usually high enough to blow a mains fuse or trip a circuit breaker.
Very small currents (less than 2.5mA) flow down to ground through the earth pin, due to leakage currents in the Class-Y RFI filter capacitors inside the PSU. This is by design. A milliAmp is one thousandth of an Amp. The pitting of the Line and Neutral pins is due to currents of roughly 60A (60,000mA).
Any marks you see on the Earth pin are probably down to normal contact wear, but scorch marks will occur on the two shorter pins due to arcing, if you plug and unplug "live". As for high contact resistance, yes it can cause arcing which in extreme caaes can cause a fire.
You'll get conflicting advice because people have different opinions, especially on an open forum. I'm an electrical and electronics design engineer. Other people have only your safety in mind and may recommend caution. Some lay folk are well-meaning but mis-informed. Make up your own mind.
It's easy to misinterpret photos, but yours seem pretty clear to me. If it doesn't fizz and crackle with a new mains lead, I'd be happy to continue using it.