It is reasonable to say that, as a social phenomenon, there is a current of thought characterized by shared beliefs, texts, history, heritage, assumptions about Islam, and worldview that can be called "Sunni Islam", even though people who identify as Sunni often differ significantly in their views.

While the existence or non-existence of intangibles, such as religious groups, can be more challenging to discuss than the existence or non-existence of physical objects, it doesn't make sense to say there is no such thing as Sunni Islam given that there is a general consensus that it is a "thing". 

A similar example is with respect to Christianity. It is reasonable to say there is a current of thought characterized by shared beliefs, texts, worldview, etc, which is called "Christianity", even though there are many differences between Christians regarding theology and practice.