Actually, nope. The Netherese stole nothing from the elves (unless you believe the elven fops who pretend it is they who created the Nether scrolls, but then I have grandma-incompatible words to say to you). They didn't built mythals either.
It's just a confusion. The Netherese created things called mythalar, which are a different thing. Mythals are made by weaving several dweomers together, until you get a cohesive whole that can sustains itself indefinitly (but can still be damaged by magical assaults). These mythals have a number of effects on all the warded zone, usually designed to make life more comfortable (automatic feather fall for everyone, for example), or for a better protection (teleportations are blocked, for another example).
Mythalars, on the other hand, are merely an endless energy source. They were created through magical fusion of negative and positive energy, creating a steady flow of raw magical energies, used to power a variety of quasi-magical items. These items were very cheap and easy to craft, but lose all power when beyond the reach of the mythalar's energy flows. Mythalars were notably used to sustain the Netherese floating cities, maintaining the levitation effect, as well as to provide the citizens with an Eberronnian level of magical facilities. (Eberron uses manifest zones, artificers, and magewright to get the same effect.)
So, you see, it's not the same thing at all. In fact, it is possible to have both a mythal and a mythalar at the same place, although I do not think it has been done.