That information is not correct.
Axolotls are always cannibalistic, although the word 'cannibalistic' makes people think of detrimental, continuous behavior. Axolotls hunt generally by sensing movement of their prey, in the wild they are fairly solitary and not needing to be in groups. You can imagine when put in a tank in close quarters with other Axolotls accidents could happen, when one passes another it could be hungry and mistake it for food and snap. This is where limbs are lost.
My girl friend has had up to 6 in a tank at once, they are all adults all 1-2 years old. They have all been in the tank together for years. I've watched them myself on the odd occasion grab another's leg while feeding on pellets from the bottom of the tank, its never resulted in loss of limb, they release fairly quickly as the other Axolotl pulls away.
While I rear larvae, I have to separate the smaller ones so they don't become a bigger ones dinner. Then this allows even growth since larger juveniles can go through the food source a lot faster when left to feed themselves.
I think juveniles tend to suffer from loss of limb more because their limbs are not as thick and sturdy as an adult, and often people keep Axolotls of different size together which makes it a lot easier for a limb of a smaller Axolotl to be damaged.
Its down to feeding habits to stop these accidents happening. You can spread out the food, feed Axolotls in different areas. Also making sure the Axolotls are always well fed, stopping any temptation for them to snap at each other. Also the longer Axolotls are in a tank with each other, the more aware they are of their tank mate(s).