the variable redeclared in the outermost block of a substatement is a

The statement in stmt.stmt#stmt.pre-5 explicitly says:

[Note 2: A name introduced in a selection-statement or iteration-statement outside of any substatement is in scope from its point of declaration until the end of the statement’s substatements. Such a name cannot be redeclared in the outermost block of any of the substatements ([basic.scope.block]). — end note]

The key term here is outermost block which is defined in stmt.block#1:

A compound statement (also known as a block) groups a sequence of statements into a single statement.

compound-statement:

{ statement-seq opt }

…

A compound statement defines a block scope.

So stmt.stmt#stmt.pre-5 is essentially saying:

if (int a = 0) 
{  // outermost block
  int a;  // so ill-formed  
} 

but

if (int a = 0) 
{  // outermost block 
 {  // inner block
   int a;  // so well-formed 
 }  
} 

The same rules apply in your example with the block introduced by the nested if statement.