Reduced recoil and great sectional density are also main benefits of the 6.5. Because of the sectional density, a 6.5 can achieve the same penetration as a heavier 308 bullet, which enables equal penetration with less recoil. There’s a reason why 6.5 Swede is effective against moose.
But regarding the powder burn, that’s not accurate, and it’s also moot. 99% of the powder in both burns within the first few inches. Just because the bullet is still accelerating at 18” of barrel doesn’t mean the powder is still burning. It just means that the massive chamber pressure is still a heck of a lot greater than atmospheric pressure and so the bullet will continue to accelerate.
What you’re probably talking about is pressure within the barrel and how it is applied to the bullet. For the same chamber pressure, there will be more pressure applied to a 308 bullet than a 6.5 bullet, because of the surface area of the bullet. So, given an equal bullet weight and equal chamber pressure, the 6.5 will take longer to accelerate to the same velocity as the 308. That becomes less of an issue with normal bolt action rifle lengths. But it does lend the 308 to lose less velocity as you chop the barrel down.
But why I say it’s moot is because even with slightly less efficient utilization of the powder, a shorter 6.5 CM (16”) will still have a greater hit probability than a longer 308 (20”). The main advantage of 308 is energy, whether it be used to punch through wild boar bone plates or to break apart concrete cover with a machine gun.
USSOCOM has been playing with 14.5” 6.5 CMs to great effect. Small enough to maneuver in tight quarters, but with the external and terminal ballistics of a 24” 6.5 Grendel (if Grendel could shoot 140s). 6.5 CM does pretty well in a short(er) barrel.