Every one of the people in that building needs the same amount of food they did before they moved in.

Same amount of food yes. But if a mathematician was just sitting on the sidelines before this new field, they may decide it is more productive to trade time they spent on preparing food for more time to spend on math.

That's just moving the food job somewhere else.

It's not clear that this thread is about. Jobs a weak approximation of utility creation + equalit of the distribution of that utility.