The difference, by the numbers
They have no ingredient in common: the Treacle has simple syrup, Angostura bitters, dark rum and apple juice, the Whiskey Smash sugar, water, bourbon and mint. In parts, a Treacle is ¼ syrup, 2½ rum and 1 apple juice; a Whiskey Smash is 1 sugar, 1½ water and 6 whiskey. By volume, the Treacle is 67% spirit, 26% mixer and juice and 7% syrup and sugar; the Whiskey Smash is 71% spirit, 18% mixer and juice and 11% syrup and sugar. Different bottles, similar balance: that makes them close cousins on the ratio map.
The Whiskey Smash is the stronger drink in the glass (25.4% ABV against 22.8%) and holds more alcohol (1.52 standard drinks against 1.17). The Whiskey Smash has more calories (about 172 against 136), with about the same sugar (5.1 g and 6.0 g). Both are built and served in a rocks glass.