He's right you know.
Wonder how the thread went? There are at least three pages. If anyone can be arsed to read them and précis, please report back.
There is a purpose to having tyres with ‘tread’ on the shoulders ( it’s not tread, it’s not there to enhance grip, or move water ) with the arrow pointing in the correct direction. It’s there to make sure that the manufacturers ‘power’ spec is applicable. The Q.A. compliance people don’t like it if a tyre’s spec can’t be matched in the real world. The ‘tread’ is an aerodynamic / fluid dynamic device, to ensure any aero drag caused by the tyre is minimised. Air against air / water against water is less draggy than rubber against air, or rubber against water, so the ‘tread’ is there to trap air / water as the tyre rotates, which reduces drag, and gives the manufacturer something which helps them ‘prove’ their figures. With a slick tyre the arrow points so that the tyre rolls in the direction that the tread was laid onto the carcass. The boundary between the carcass and the tread is not actually perfectly uniformly flat, it’s ever so slightly ‘pear shaped’, in order to keep the tyres performance as ‘per spec’ the tyre should roll from the bulb to the tail, on each rotation ( this is also the case with ‘patterned / treaded tyres, but the pattern overrides the tread / carcass interface issue with patterned tyres) All of this is of little real world consequence, but compliance / Q.A. are a pain in the butt, and need stuff like this to keep them quiet