I know nothing about letting lead cure, but I know a little chemistry. If there is a reason to cure them I've never heard of it, nor is it discussed in my Lyman's cast bullet handbook, and what ever reason there may be isn't what your friend described. Yes when you heat metal the little bits, atoms, get to moving like hell. That's why the stuff turns to liquid. But once it's cooled they settle down and are locked in place. That's why it turned solid again. Concrete on the other hand involves a series of chemical reactions, and they don't stop for a long while. Thus it's not right until the reactions finish doing their thing.
Not saying there is no such thing as curing bullets for some benefit, but I can't think what the benefit is. Are we really talking about heat treating?