Every person has two copies of the BCMO1 gene. But, about 45% of the population carries at least one gene variation that reduces BCMO1 enzyme activity, resulting in significantly impaired ability to convert beta-carotene into retinal.
Basically alot of people can’t use plant vitamin a and need the real thing.
A lot of nutrigenomics is basically a scam to scare people into using certain supplements. For instance, it is basically implausible for people to have vitamin A deficiency despite high beta carotene consumption, regardless of BCMO1 polymorphisms, and the same with MTHFR and folate.
A lot of diet ideology is starting from a conclusion of what foods to eat and then working backwards to find evidence to support it. Be very wary of claims that a certain diet is the one perfect way of eating.
It isnt abolished activity, it is lowered activity. You need so little retinoids and if something is always around (BCO1, convention has changed to BCO1 and BCO2) you make enough. The step you are missing is an association between those variants and insufficient levels of retinoids. I am unaware of a study that demonstrates such.