Okay, think about this: in the german language, "ei" is pronounced like the word "eye" or simply the word/letter I. Why german? Well, I am a native german speaker.
Also, a more valid reason is this: Sauerbraten is a german dish. Literally translated it means "sour roast", so I think using german pronounciation is a legit assumption.
The second thing is that in german, the "U" can be pronounced in different ways, for example like the "OO" in some words. Examples for the pronounciation I mean would be "cool" or "mule".

Taking both together, it would mean that "eihrul" sounds like "I rule". Coincidence? I think not.

Eihrul is AFAIK the current lead developer of Sauerbraten and Tesseract. Eihrul hosts important Cube2 sites, for example sauerbraten.org. Eihrul is the one who is responsible for the global authcodes. Eihrul hosts the masterserver.
If he wanted, he could have a tremendous impact on the community.
That all means that eihrul indeed rules. Perhaps his nick was meant to reflect that?