This Nemetes-theory seems to be very rare. It has indeed been brought up already in the 19th century. But until today, the etymology in nearly all publications is explained by a Slavic verb for mute. See for example this work on foreign words in the Slavic languages from 1864 (in German). It says: The connection to Nemetes has to be rejected, from linguistic reasons as well as from reasons lying in the matter itself (sorry, don't know a suitable translation for sachliche Gründe, here apparently meaning historical reasons). The Nemetes are a people of the first three (?) centuries AD on the Rhine. The Slavs came from the east, to Moravia in the 5th century, Bohemia in the 6th, and it was not before the 7th century that they came as far as northern Bavaria. The Nemetes are not attested any more at this time, and have never been attested somewhere further east. It is nearly impossible that the name of a small, long vanished and romanized tribe, which has never encountered slavic tribes, gave its name for the Germans in general. 
 ::::The sources given for this theory are few, I find them mostly in the 19th century. One article in Polish is (if I extract this right from the Polish explanation on the web page cited) written by a biologist (!), and I found only one reliable source mentioning this theory (Journal of Indo-European Studies 1974) 
 ::::Regardless of being a popular explanation, the derivation from a word for mute is the standard explanation in nearly all etymological dictionaries. We have to trust the specialists in etymology that they have considered this question for at least two centuries and have come to a conclusion which is the most probable at the state of our knowledge. It is not acceptable that this Nemetes-theory is presented by the Wikipedia as the only explanation or even as the main derivation.