She said you can’t earn it.

Everyday, people grind their own flour.

Then I build a mill, and allow them to use it in exchange for a part of their flour.

Is my wealth not earned?

You would not get a billion dollars from that. Someone else would see your mill and make their own mill. A thousand other people, in fact, if there was a billion to be made, so each would get a million, or less. The intense price competition would also bring your profits to near zero.