In «mā anā qultu hādhā», negation precedes the subject expression, and the verb comes afterward. This brings the identity of the alleged speaker into focus.
ExpressionFocus
مَا أَنَا قُلْتُ هَذَاWas I the person who said it?
مَا قُلْتُ هَذَاDenial that I said it, without the same necessary contrast of agents
The latter does not automatically deny that anyone else said it. The distinctive interest of the first arrangement is contrastive denial of attribution.