I think this is based on another airline's cabin crew union arguing and winning a case based on the premise that holiday pay should be calculated on what their members were paid on average during the year, their salary package if you will, and that of course includes sector pay.
Their holiday pay had been based on basic salary only, and this in many cases is only half the "normal" pay received when flying the line.
I don't know about the tax perks, if any, they had attached to sector pay. Hopefully in this case, the union has calculated that even if they lose the tax breaks, they will still be benefiting overall.
Just to confirm, easyJet pilots are not pursuing this.