The difference, by the numbers
Both have Angostura bitters. Only the Brandy Old Fashioned has sugar, orange juice, cocktail cherries, Cognac and lemon-lime soda; only the Treacle has simple syrup, dark rum and apple juice. In parts, a Brandy Old Fashioned is 1 brandy and 1 lemon-lime soda; a Treacle is ¼ syrup, 2½ rum and 1 apple juice. By volume, the Brandy Old Fashioned is 48% mixer and juice, 47% spirit and 5% syrup and sugar; the Treacle is 67% spirit, 26% mixer and juice and 7% syrup and sugar. That makes them close cousins on the ratio map.
The Treacle is the stronger drink in the glass (22.8% ABV against 17.1%), but the Brandy Old Fashioned is the bigger drink, so it holds more alcohol (1.43 standard drinks against 1.17). The Brandy Old Fashioned has more calories (about 190 against 136) and more sugar (12.4 g against 5.1 g). Both are built and served in a rocks glass.