Quotes like this are common but don't tell the whole truth. Yes, it's true that BBI got these results using commercial handgun loads. However, if one handloads and uses magnum pistol powders like H110 or W296 or Lil'Gun the results are surprisingly different.
Consider this - I own 2 Rossi 357mag leverguns, a 24" rifle and a 20" carbine. I shot the exact same ammo out of both, a full power load 158grn 357mag using H110, SPM primers, and a Zero JSP bullet in a Starline case with an OAL of 1.580".
I chrono'd both of them (and two from friends) at the same time on my range which sits at 6,100ft asl on an 84° day. The results were: my 20" Rossi carbine - 10 rd average of 1,789fps, 24" rifle - 10 rds average of 1,822fps. The 4" longer barrel produced 33fps higher velocities.
Then I tried my neighbor's 24" Rossi 357mag rifle to compare velocities with my same ammo. It produced 10 rds average of 1,827fps. I also tried a friend's 20" Henry Big boy in 357mag with my ammo and got a 10rd average of 1,783fps. Consistent results showing that the extra 4" of barrel produced over 30fps higher velocities from my handloads.
Conversely, I also own a 24" Uberti 1873 Special Sporting rifle and a 19" Uberti 1866 Yellowboy carbine, both in 45 Colt. Due to their weak actions, I only load standard level loads using Unique powder. The standard level pistol load produces higher velocities in the 19" carbine than in the 24" rifle.
My conclusion is that while I believe BBI's data, they used ammo formulated to burn quickly in short, pistol length barrels while my handloads with H110 were more able to use the extra length of the barrel to add velocity. This is also supported by Buffalo Bore's 158grn Heavy 357mag loads where they claim 2,153fps from an 18.5" Marlin barrel.
It's not the caliber, it's the formulation of the load that makes the difference. Once can load for a short 5-8" pistol barrel or one can load for a longer, 18-24" barrel.