The word kabīrah commonly includes grave sins that do not constitute unbelief. In a particular polemical interpretation, it is also construed as referring to unbelief as the most complete form of grave offense. The ordinary category and the special argumentative usage must not be conflated.
An objection asks why a plural is used if the reference is to unbelief. The response argues that a broad category can have several forms and be mentioned in the plural.
This is a specific interpretive argument, not a rule that every occurrence of “major sins” must mean unbelief alone.