When you rely on only food to get your B12, you need acid in the stomach
to help break the protein down to get to the B12. (there is typically very little there).

When you use a supplement you are taking way in excess of what you would typically get in food, so that passive absorption has to be how it gets in, (without instrisic factor etc)...so 1000mcg may yield 4-5micrograms.

You will absorb from the supplement without acid being needed.