Great question! That song only plays that high for the last few notes. You can play that section down an octave (3rd, 4th and 6th fret) instead of 15, 16 and 18. Basically subtracting 12 frets.
Most songs on this site do not move beyond the 15th fret as it’s just too hard to reach higher with the body blocking hand movement beyond the 15th fret.