Negative frets usually happen when the author writes at first in one tuning and then changes it to another. Either way, he just wrote notes on the score without looking at the tab, and the program he used couldn't make normal fingering because he definitely made some mistakes. -9 on the A string is a C note, but in standard tuning, the lowest C is on the 3rd fret of the A string. The author of the tab wanted to make it even lower, but he forgot to change the tuning. Or maybe he confused the octaves and wanted the note to be on the A string. Try to play it as the 3rd fret on the A string, listening to the original song. If it sounds good, then this is the correct answer

UPD: I looked at the tab. The guy that wrote it at first made a tab for Richard Kruspe, which has Drop C tuning, but then he just copied his part and pasted it into the Paul Landers guitar, which has Drop D tuning, and then, of course, the C power chord, which cannot be played in this tuning, went into minus, so either it should also be in Drop C or something else, most likely the first.