Exploding or alcoholic soda
This recipe is listed under the section for fermentation, together with beer, wine and mead. The section starts with the sentence "Wine, beer and traditional sodas all depend on yeast to ferment sugar into alcohol and generate carbonation". 

I don't know enough about the history of soda to know if early sodas were alcoholic. Or rather, I am quite sure that there were alcoholic, fermented, carbonated drinks long before what we call "soda" today existed, but I don't know if they were called soda. 

Whatever the language problem is, this recipe is definitely intended to produce a low-alcohol beverage, comparable to beer. If you want carbonated syrup, you should buy a carbonating machine. These take a bullet full of CO2 and press it into the drink base you have selected. 

As for the too-strong carbonation, this is probably due to the vague term "room temperature". Yeast growth speed depends on temperature. Because it is an exponential growth, even small changes in temperature can lead to vastly different results. If you want to repeat the experiment despite the alcohol production, try better controlling for the temperature. As I don't brew, I can't tell you the temperature for optimal carbonation after two days, you will have to find it out by yourself.