The AUH-PER flight shows a block time of 11h20min but in EY's book this is not long enough to warrant lounge wear or slippers.
EY business class does not provide lounge wear or slippers, regardless of the flight duration. I understand you're disappointment as I had the same experience when I flew. Of course they are provided in First Class and are very comfortable.
However, the increase in quality from Business to First (on B787) is not great enough to justify the expense. The food is the same quality, as is the champagne and the seat is only roughly 10-20% wider and the same length.