> Animals don't synthesise B12, bacteria does.
Yes, that's technically correct.
> Animals cumulate it from eating food grown in soil.
No.
Animals provide a hospitable environment (gut) for said bacteria to exist and synthesize. So while technically the animal's cells are not the ones directly synthesizing B12, the bacteria and animal gut are working in harmony to do so.
These facts are tangential to my point. The commenter I responded to said that people don't need to eat animal products to live at optimal health, which is laughable, and this is an incredibly clear example of why. A quick google search will illuminate the dangers of B12 deficiency.
And one more thing to consider, if humans coevolved with the animals they eat, how can we be so certain that simply supplementing a few vitamins found in animal products is a sufficient replacement? We hardly understand human metabolism at its most basic level, it's quite a stretch to think that we've cracked the code on the "essential" nutrients we would have received from consuming animals.