Citation please? I've looked into this subject pretty intensely over the last decade or so and know of no controlled studies (or even poorly done observational studies) that demonstrate this in the least. The only place I've ever even seen this claim is on vegan advocacy forums as unsubstantiated claims or in studies of nutrition absorption problems as a general factor in the elderly. The amount of animal meat needed for full dietary B12 is actually shockingly small. A few grams a week. And it's water soluble so you just piss out any that's not used.
On the contrary I've read several studies that show that in the study populations, up to 92% of the vegans in the population were B12 deficient.
In another a little over 50% of vegans were deficient, while less than 10% of vegetarians and a single omnivore were B12 deficient.
I know of no studies where the studied, randomly sampled, vegan population was not majority B12 deficient.
In other studies that look for it, elevated methylmalonic acid (associated with B12 deficiency) was found in the vast majority (over 80%) of vegans while only 5% of ominivores and 68% of vegetarians. In the same study almost 70% of vegans were diagnosed with Hyperhomocysteinemia (caused by sustained deficiencies in B-vitamin groups or chronic alcoholism) while less than 20% of the omnivores were.
The most effective and reliable treatment for Hyperhomocysteinemia is a regular omnivorous diet of meat and eggs with a cure rate of nearly 100%.