Before I answer....I'm an atheist and history has proven to me beyond doubt that the bible is very human and not of deity origins.....
That being said, note that Watchtower teach that nobody was anointed until pentecost 33, AFTER jesus had died. So nobody at that upper room, consuming the last meal was 'anointed'.
Simply ask an elder to read the verses pre "keep doing this in remembersnce of me" and ask them if we should in any way change or alter how it was done? when they say no.... then mention that none of them were anointed, why do WT add that rule? 'He' said keep doing THIS.... not anything more or less.
snare x