«أنا سمعت» can reinforce that the speaker himself heard something without necessarily denying that anyone else did. Added qualifiers such as «وحدي» and «لا غيري» require their own semantic analysis.
ExpressionPrimary force
أنا سمعتAffirms the speaker's own hearing
أنا سمعت وحديStates hearing in solitude
أنا سمعت لا غيريExplicitly excludes others
Thus subject emphasis and restriction or exclusivity (qaṣr) are not automatically interchangeable.