The similarities are there and I wouldn't dissent them. If you focus just on the differences, even different kinds of apples are not similar anymore.
However, a direct inspiration seems very unlikely. If Avenged Sevenfold did make use of the guitar riff, they would've openly said so, or they would've incoporated the real sample before playing it with their guitars. While sampling doesn't require credits, in such a singular case as this (sampling in Metal isn't common), at least I'd expect there would be a note about that in an interview or so (though, I haven't seen any interview by them, so I can't back up my statement).
An indirect inspiritation could've occured more likely, however. I don't know if the writers of the song were passionate UT players (they would be old enough). If they were, Razorback could have been stuck in their head unconciously while writing the riff. When I write music, I sometimes detect years later, that it is pretty similar to something I knew before - but it didn't come to my head at the time when I wrote it.
The other way round, that means Razorback has a sample of Avenged Sevenfold, is without question, a false assumption as the difference in the release dates is too big. Still, it could be possible that Razorback samples a guitar riff from a rock song. But I think it's especially recorded for Razorback.
(there's no hint about anything else... http://www.futurecrew.com/skaven/tracker_music.shtml