... or how you deal with a blocked cat! (laxatives won't work).
The polarity reversal could work in a simple wet cell if the plates are of the same design and material, but note it is fully discharged first which is intended to make the plates the same. Then charging the opposite way reverses the reversible reaction. It may work by breaking down a layer of solid sulphate that otherwise doesn't dissolve readily. It won't help if there's a structural fault. You could then revert to the original polarity by fully discharging. The Russian guy was only using 12V charger, so no high voltage. Uncle Tony's welder is essentially a transformer/rectifier converting 120V to something like 30 or 50V DC (did he say?), so > 12V but no polarity reversal. So it's just like overcharging with a bad regulator and probably works by the high rate of gas formation blasting off solids from the plates. It may be rather like how an ultrasonic cleaner works. the test would be how much material ends up on the bottom of the cell. The old trick is to drop a lead acid battery on a plank to shift the sulphation. Not going to work for AGM. Epsom Salts are claimed to dissolve them and restore a battery.
p.s. Uncle Tony's battery was apparently a new battery that had been discharged and left for a few years. Try it again with a 5 year old battery that won't hold a charge, because it will likely have a lot more degradation and sulphation of the the plates, maybe even shorted in a cell.