Put it this way, water is going to be either corrosive or scale forming.
An espresso machine runs at a variety of temperatures. From room temp, to 105c in one boiler (approx) to approx 125c in another. It’s not possible to feed a machine with water that will be balanced across all temps. So you can have water that is neither scale forming or corrosive at 105c, but scale forming at 125c. Or you can have water that will be neither corrosive or scale forming at 125c, but corrosive at 105c. It would be a bit foolish to try and supply a service boiler with such water as you’d have to completely empty it out and refill every day, so you’re better off either using water that lacks hardness and is simply corrosive across the whole machine, or using water that is balanced in the brew boiler but scales in the service boiler - meaning you have to descale it (not usually an issue, it’s the brew boiler that’s a pita).
Also worth noting that ‘corrosive’ is used loosely. It’s according to the Langelier index, and I’m not sure it is entirely appropriate to use it when talking about water remineralised to have alkalinity to stop it from becoming acidic. So you could do what I and others have done or still do and that’s remineralise distilled with sodium bicarbonate. Or you could try adding some hardness in and keep an eye on scale in the service boiler….up to you. OR you could go for Hardness and alkalinity from non-bicarbonate sources (which TWW may do).