«لَا مَحَلَّ لَهَا مِنَ الْإِعْرَابِ»
It has no syntactic position in the surrounding construction.
The clause is not serving as the main sentence's predicate, adjective, circumstantial clause, object, or another case-bearing constituent. Its own words remain grammatically organized, but the inserted clause does not occupy a syntactic slot within the host construction.
The distinction is between the external syntactic position of the whole parenthesis and the internal grammar of the parenthetical sentence itself.