That is a simplistic explanation.

1) if a person has intact instrinsic factor still (often destroyed in autoimmune disease or by stomach acid lowering drugs), small amounts of oral B12 may still be well absorbed. MAY

2) But when intrinsic factor is NOT working at all, then large doses need to be used on a daily basis so that the minute amount will pass thru the intestinal lining under ideal conditions. This is called passive absorption since the carrier--intrinsic factor--- is not helping it. Tiny amounts of B12 eaten thruout the day--will not be absorbed at all if intrinsic factor is broken.
When this happens one needs at least 1mg of B12 orally daily on a EMPTY stomach. If one is very deficient, 5 mg daily is better until levels normalize then 1mg daily thereafter, is often sufficient for many but not all people.

I have yet to see a vegan site that discusses B12 correctly and accurately. I have seen some who claim B12 can be derived from vegetables! (which is a terrible error and very misleading)