I initially thought the same, but I'm willing to bet that 2525 was trying to say that the burning of the remaining powder was unable to sustain the same pressure as the volume expanded due to the now-moving bullet.
On the other hand, I think he may be incorrect in thinking the bullet is no longer accelerating once the powder has burned completely (or as completely as it's going to burn; there is usually up to 15% unbunred powder.) The bullet will continue to accelerate as long as the pressure behind it is greater than the resistance of the bullet against the barrel. The Delta Vee will be reducing as the pressure behind the bullet drops, but it's not going to be negative.
