Why do we need to use virtual ~A() = default; instead of virtual ~A() {} in C++11?

The comment is not correct.

Both:

virtual ~A() = default;

and

virtual ~A() {}

are user declared. And the implicit move members are inhibited if the destructor is user declared.

[dcl.fct.def.default]/p4 discusses user-declared and user-provided special members:

A special member function is user-provided if it is user-declared and
not explicitly defaulted or deleted on its first declaration.