It is in list context because, and only because, the assignment operator is aassign and not sassign. I.e. since you're assigning to an aggregate the RHS will be in list context. It has nothing to do with the parenthesis or the comma. ( 1 => blah() ) can just as well be in scalar context in which case blah() will be called in scalar context.

  $a = ( 1 => blah() );

  __END__
  wantarray =
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ihb

Read argumentation in its context!