That is still technically incorrect and is refuted by observing healthy old vegans that consume supplements from cultivated-bacteria only for decades.
Those bacteria also develop in some animals digestive system as you already know, and eating those animals has been for a long time our main source of b12. The other (minor but non trivial) one has been non-washed fruits and plants human consumed during millennia, and that’s still how grazing animals ingest a bit everyday. The non grazing animals are widely supplemented with cultivated-b12. We’re producing around 80T/year for that which isn’t much we animals only need around 10mg/year. I'm not discussing the "non-natural" qualification of supplementation, just the fact it's today a very white practice to live an healthy (human or non human animal) life by ingesting only bacteria sourced B12.
> we've cracked the code on the "essential" nutrients
Yes we did, and the last of that nutrient has precisely been vitamin b12 which led to two Nobel prizes : https://pubmed.ncbi.nlm.nih.gov/23183296/