Using the most literal definition, yes, it would create a negative externality for others within that insurance pool. But when we talk about negative externalities in this kind of context, we generally mean “negative externalities that merit public intervention”.
If a factory emits harmful particulates into the atmosphere, it worsens the air quality for a much larger population and is much harder to avoid. That’s a point when it would be a reasonable call for public intervention.
Anyways, some fat dude that you work with will probably cost more to the company that’s insuring his healthcare, but you can’t definitively say that soda is the primary culprit for his fatness. Singling out soda for a tax is discriminatory and wrongheaded because any number of things unrelated to soda could be responsible for his health problems.