This is how I look at it ...
The site you pointed to is a lab in a university. Labs want things always done in one particular way so that results can be duplicated. Add to that the number of people that are for animal rights or against animal testing and you come down to a lab setting only one method (that is viewed as humane) to dispose of their animals.
Most labs look at 2 types of euthanasia as humane. Co2 and severing the spinal cord. I dont like the spinal cord method ... because if you mess it up you have a upset hurt animal, and it takes a bit of practice to get it right.
Dry Ice is deemed inhumane as you cant accurately control how much it will sublimate, instead of a quick knockout and death, they can lay their gasping for a few minutes, and if the Co2 isnt dense enough they can recover (with the effects of Co2 poisoning). Dry ice can also be quite dangerous ... in addition to contact burns/frostbite it is continually offgassing and can cause an airtight container to burst or rupture.
Bryan