Last I checked none of those things are fruits or vegetables. Suggesting they were enough to get all necessary vitamins and minerals was simply inaccurate and bad health advice.
When I Google "mushroom B12" it seems to suggest mushrooms have almost none.
People used to get B12 by eating meat (which I use generally to also include seafood).
People used to get B12 from the bacteria in the soil and the plants we eat. Not, anymore.
So, yes, you could eat some meat. B12 is present in very high concentrations in a lot of sea food.