I put this one down as either misinterpreted info, or wishful thinking. All previous fast zooms have not outdone primes in terms of max aperture in their equivalent range. Take 24-70/2.8's or 70-200/2.8's - plenty of faster primes covering those ranges.
The 18-35/1.8 from Sigma is a crop camera lens, covering an equivalent of 29-56/2.9 on full frame - direct equivalents are the FF 24/1.4 and 35/1.4 lenses, which outdo it. Nothing fast exists around the 18mm mark, so it is very impressive for crop. But when compared to a FF system, just like a 24-70/2.8, its nothing special compared to FF/fast primes.
How is it possible to make a lens a whole stop faster than any prime in its range, and also make it a zoom? I know its a limited range zoom, but really?