Wrong.
1) ALL of the powder is consumed within the first few inches of barrel ahead of the chamber. This has been proven for decades.
2) Barrel twist rate and length are not actually related. A shorter barrel, with a given twist rate, is no less accurate than a longer one with the same twist, if all other factors are equal. In fact, a shorter barrel can sometimes be inherently more accurate, because a shorter barrel of a given diameter is automatically stiffer....and thus does not suffer as much from barrel whip.
What IS lost, with a shorter barrel vs. a longer one, is some velocity. That does NOT translate into a loss of accuracy, in most cases. Certainly, less velocity will produce more bullet drop, but that does not necessarily produce less accuracy. It can be true that a given load does not work as well in a shorter barrel (vs. a longer one)...but that is easily addressed by tweaking the load.
My 18" .223 barrel is every bit as accurate as it was at 22", before I cut it down. It even likes the same loads. The ONLY thing that changed was velocity and therefore, bullet drop, which was easily compensated for by scope adjustment.