I think people are reporting this relative to expectations rather than absolutely: they assumed it woudl be slower due to the lower cost and lower power consumption but the results defied that expectaion.
$250 is not exceptionally low cost for a netbook. Furthermore the power dissipation of the chromebook under load is only slightly better than a comparable atom system [1]. The idle performance is better, I will give them that. Still, the death of x86 in the face of ARM for performance-sensitive environments (even when power consumption is a factor) has been greatly exaggerated.
A15 is 40-65% faster than Atom and uses half the power consumption, according to your link. So yes, I'd say that's pretty impressive at its level.
Also, this specific Chromebook is higher quality than those netbooks. It's more like an ultrabook in terms of design. Not to mention that it has twice the battery life of those netbooks under normal usage.
[1] http://www.anandtech.com/show/6422/samsung-chromebook-xe303-...