I think the initial thought relates to cooling the exhaust and by increasing density through the cooling backpressure would be reduced. However, someone that is much better with steam tables than me would need to evaluate whether or not the denser exhaust gasses would more than counteract the expanded water volume.
As a side note - I still can't come fully to grips with the way the term backpressure is used with regards to exhaust. Backpressure in the exhaust does not help at all. If the real object is scavenging - then it is exhaust gas velocity not backpressure that produces this low end toque benefit. To the extent that a larger header primary reduces velocity of the smaller volume of exhaust gasses at lower rpms then scavenging suffers - adjusting the diameter of the header primary to maintain a good exhaust gas velocity while not overwhelming the system with backpressure is where exhaust tuning comes in. Off course the pulling effect of exhaust gas velocity is relevant to maintenance of scavenging at the header collector all the way until the muffler tip so tuning needs to be done all the way through. But it is the maintenance and balance of velocity not backpressure that is beneficial.
Once exhaust gasses have gone through the turbo they no longer have any significant effect on scavenging. Ideally you want to decrease back pressure as much as possible in order to increase the pressure differential on the turbine increasing its effectiveness. In on car applications you generally would not be able to utilize a fully optimal design, since you have to evacuate the exhaust gasses quite a distance from the turbo. So you still want to maintain exhaust velocity sufficient to assist pulling the following gasses through the system and keep back pressure low. Generally post turbo you want to reduce backpressure at all operating points as much as possible without totally stalling velocity until the exhaust tip.
Thoughts from someone who almost understands the science of this stuff but always remains ready to be corrected and learn.
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