I think any discussion concerning 'what is true metal' is only going to end in parody. Perhaps this is because the term 'true metal' implies any variance of metal music that does not fit within the boundaries is 'false'. Who wants to admit to listening to false metal?
Arguing for the sake of discontinuous genres is probably pointless, and not particularly enlightening - unless someone is looking for a rough comparison of sound between bands.
That said, I do not think the inclusion of every band that is loosely related to metal at Wacken is a good idea. For the record, I think that Mendeed is a questionable addition, since this is what I hear when I listen to them:
Metalcore style vocals, shouty bits with cliche melodic parts; a respectable amount of double kick, but undeserving of the death metal comparisons; Dragonforce based guitar melodies giving the pretense of power metal, except with basic riffing which is very simple and accessible - essentially a collection of elements which making this band very palatable for those beginning a transition into extreme metal, or with a taste for the more popular edge of metal.
After listening to some tunes off of their previous offering, it is clear that they were substantially more metalcore than their upcoming release, and are not outside the trend of popular metal, changing as tastes do. Therefore I find it unlikely that many fans of extreme music; black, death doom or whatever are going to be particularly impressed with Mendeed.
There are so many great modern bands at the moment, it beggars belief that Wacken is booking so many trendy metal bands instead.