In reviewing the water profiles used in those beers, its appears less likely that alkalinity is the source of harshness percieved in the beers.
The profile used for the Saison is balanced ionically and the residual alkalinity is near zero. Conversely, the profile used for the Dubbel is not balanced ionically, suggesting that there is an error in the reported ionic concentrations. Additionally, the residual alkalinity for the Dubbel profile is an astronomic 256 ppm, which is far too high for good mash performance and taste perception. That high residual alkalinity could be a contributor to harshness perception and poor mashing performance in any beer.
The indication from the brewer that both of these beers were percieved as harsh suggest that there is some other problem impacting the taste perception.
I am curious why the brewer used the water profiles he listed. The Saison profile appears somewhat appropriate for that style, but the sulfate concentration is a bit high for that style and the slighty elevated chloride concentration can contribute to harshness in conjunction with the sulfate. I would recommend significantly reduced sulfate and chloride for this style.
Understandably, the brewer used a more alkaline water profile for the darker Dubbel style. I agree with that change, but the level of alkalinity used in that profile was far too high. Assuming that the hardness remained as he shows, the alkalinity should be reduced to about half the concentration that was used. Additionally, the sulfate concentration is still too high for a style that is not hop-focused and the chloride concentration in conjunction with the sulfate can also produce a hashness perception.
I am in agreement with Gordon regarding the overuse of water profile adjustment with some waters. He and I are currently working on a project for BJCP. There are some water sources that just are not going to provide a good result, no matter the adjustment with minerals or acids. Its sometimes best to just start with a distilled or RO source and add only the desired minerals.