I imagine it would be the same reason that K&N and other free-flowing air filters claim more power and better mpg.
I'm guessing those product manufacturers have an incentive to say whatever they can get away with to sell merchandise.
There's probably some truth to the idea that this specific modification can produce more power; like just doing napkin math if you achieve 97kpa (14.06psi) at WOT outside the throttle body with the OE intake assembly and 100kpa (14.50 under the same conditions with the airbox/filter/horn being discussed here that's pretty similar in effect to having a 0.44psi boost which might nudge your 395hp engine just barely over 400hp peak.
But that's just the difference you measure at WOT, it might be like 99.97kpa vs. 100kpa at 1/8th throttle and the fuel trim will just adjust to compensate since it can't really differentiate between the restriction presented by the throttle blade and any restrictions occurring upstream of it. If you find yourself using 0.03% less throttle to achieve a given target acceleration (using the same total volume of air and fuel) and the engine calculates your shift point 0.03% earlier since it's measuring 0.03% less commanded throttle from your pedal, would you notice that on your fuel receipts? I wouldn't.
Not saying I'd mind a plausible five peak horsepower if it's there and almost certainly without any drawbacks or gotcha's but there's just no mechanism by which this would improve fuel mileage.