Well, I looked in the Speer #13 for 30-30 info in a 14" barrel. Speer pushed a 150 grain bullet 2171 fps out of that barrel, and I've found their velocities to be very close to what my chronograph says, with the same barrel lengths.
Now, a 357 loaded with 140 grain bullets, out of a longer 18" barrel, only went 1934 fps for the Speer folks. Lighter bullet, longer barrel, and it still can't match the 30-30 for velocity, and that means it's shorter on energy, too.
Throw in the better sectional density of the 30-30 bullets, and it gets pretty hard to make a case for equal performance. I haven't seen the Buffalo Bore info, though. Is this another one of their magnum +P loads, just in 357? Or, perhaps they feel their load gives similar performance on game, and that's the basis for the claim.