Dave, here is the whole paragraph,

"Far from being fought over trivial issues, World War One must be seen in the context of an attempt by an aggressive, militarist state to establish hegemony over Europe, extinguishing democracy as a by-product. To argue that the world of 1919 was worse than that of 1914 is to miss the point. A world in which Imperial Germany had won World War One would have been even worse."

Only a complete moron would expect it to be spelled out that Sheffield can not actually see alternative histories!
A professor and doctor of history whose life's work has been the study of that period is well placed to extrapolate upon his vast knowledge to consider the most likely outcomes.
That is what he did, and if you read his books you will see how he supports his views with hard facts.
His peers would rip him to shreds if he could not.