I think raw trem speed doesn’t count. Something like the trem picking part in the middle of EVH’s “Eruption” does not constitute “shredding” to me, and I think it’s pretty attainable to brute force as a beginner. I think Troy even showed a beginner to trem at high speeds on day one of ever playing guitar?
Something like the typical sixes pattern at about 105 BPM sounds like “shred” to me, or the main line from LTE’s “Universal Mind”, which I think is mid 150 BPM 16ths. While both of these are similar NPS, I think the sixes pattern would be in the realm of an intermediate player, whereas the Universal Mind line would be a category higher, easily (advanced / shredder in the chart).
I think the only way NPS could ever convey skill would be if it were paired with string changes per second. Something like a graph with gradient zones (I don’t believe that’s the right term, but I’ll stick with it lol).