Are you being misled by the term "bitter lemon"? This is a popular British soft drink, originated by Schweppes, which appears to be similar to the lemon tonic mentioned in the recipe. I don't know how similar the two drinks are as I have never heard of lemon tonic until today.
I'm familiar with the soft drink. I was just wondering if there were some additional bitters, e.g. Angostura, in this recipe at one point and that maybe that was why it is listed as a cocktail with alcohol.