Ay up. I just took another look at the website for the Montreal Botanic Garden, since I'd alerted them to a typo in their list of recommended roses some time back, and also asked them to add clarification for a couple of others. They appear to have done this.

http://espacepourlavie.ca/en/recommended-cultivars

As it turns out, their recommended cultivar for disease resistance is not this 1969 rose, but the 1984 Kordes rose of the same name (.../rose/l.php?l=2.5542 or KORzaun/Royal William to his friends).

So, the note about Montreal's recommendation should be removed from this rose's description page, and added to the other one. :)