And now you get to the crux of the issue. If you go back and read what is being said in those posts, specifically from Form and others, you will see that what they clearly say is that in that particular instance, the damage that was done and the way the animal reacted in that situation, was very similar to what they have seen when an animal, in a similar situation, was shot with a bigger bullet.
That is how you prove theories. You have a hypothesis, in this case that small diameter, heavy-for-caliber, frangible bullets, with impact velocities of 1800 fps or higher, will kill just as effectively as large diameter bullets of similar construction and similar impact velocities. You try to prove that hypothesis through testing and you observe the results. Sometimes, while doing that, you also find out things you weren't trying to prove. In this case it is that small diameter, heavy-for-caliber, frangible bullets, with an impact velocity of 1800 fps or higher actually kill more effectively than large diameter mono-metal or bonded bullets, regardless of impact velocity.
In the particular case of proving this hypothesis true, I believe that the evidence is sufficient to have proved that. Therefore, we can predict that if a small diameter, heavy-for-caliber, frangible bullet, impacts a game animal at 1800 fps or greater velocity, it will cause a sufficient wound channel to incapacitate said animal just as quickly as if it were hit with a larger diameter bullet under the same circumstances, thus proving the theory.
When theories are proven through sufficient evidence that we can effectively predict outcomes with a high degree of certainty, then they are considered proven science, i.e. fact. As to whether you believe it or not, is up to you. Hell, it took a couple hundred years for people to believe that the Earth circles the sun and that said Earth is round, despite the evidence to the contrary.