For what it's worth Smith & Wesson's website calls the firearm history request a "letter of authenticity". The OP is correct, the letter doesn't really authenticate anything other than how the gun originally shipped.

However, I don't know what an actual factory authentication could do that a diligent buyer couldn't also do. An in person inspection should be able to verify if the finish is original, if the grips are the original style and fit appropriately, barrel is correct length, etc... I've never heard of anyone coming up with a fake S&W. I know there were early 20th century European knock offs, but anyone with any familiarity with S&W's should distinguish one of those. They could change the name but in the end it would be same thing.