Might be to stop his fingers catching the pickup whilst picking ? ... I know Steve Vai used a bit of tape on the lower edge of his PU's to stop the plectrum digging in at the pickup . Excellent playing picking tune out of chords just like my guitar tutor showed me
I tried picking WL a few years back but mainly used the first position chords and it does work well as this version proves and judging by your skills Ecca you should be well capable of doing this one on your new L5... no excuses now