Ah, yes, those are impressive indeed, I've read about them before. As for their exact mummification I don't know, but I would place my bet on some sort of poison/toxic this in conjunction with an airtight and oxigine low tomb, could very well have such a great mummification as result. They say environmental circumstances were far from optimal for mummification, so I expect a method which involved chemicals.

When toxic levels are very high in a body, bacterias and the like will naturally get no chance and will be slowed down tremendously. When a body is filled with toxics, it will decay at a much much slower rate,

Cheers