I thought they had high alkaline water, well suited for stouts. Is this not the case?
Most of Ireland does have high alkalinity water. However there are places that do not. The southern area of Dublin is blessed with relatively low mineralized water. Guess where Guinness is located.
Of course, Guinness now uses RO for all their brewing water, so it matters less where they are located. RO treatment is probably to ensure a stable water quality for their brewing. The Dublin water can vary since they have multiple sources. Playing Russian Roulette with your brewing water is not the formula for a consistent product.