You are correct on the actual velocity's. However, the original ad meant in comparison to bullet weight the rate of velocity is higher than either cartridge. Apologies for any confusion.

The velocity of the LT cap 425 grain 20 bore bullet is 1,500fps.

303 British is roughly 2700fps for a 150 grain bullet.

256 Mannlicher is 2580fps for a 139 grain bullet.