When my neighbor first noticed them I thought they were fruit.
Google lens suggested a thousand things that were not what it was. Japanese this, or Indian that.
It does smell very much like citrus, though.
The flesh is tough and yellow but almost immediately turns dark brown. So it was too late when I noticed my fingers had stained.
Limey nut would be an appropriate name for it.
Black walnut is a weird thing.
Those are, indeed, walnuts.