What happened when I pass a nested braced-init-lists to constructor in C++?

Question

I passed an nested braced-init-lists to class constructor:

#include <iostream>
#include <type_traits>
#include <variant>
#include <vector>

class NestedInteger final {
 public:
  NestedInteger(int i) : type_(INT), val_int_(i) {}

  // NestedInteger(std::initializer_list<NestedInteger> ni) {
  template <typename T>
  NestedInteger(std::initializer_list<T> ni) {
    for (auto it = ni.begin(); it != ni.end(); it++) {
      val_vec_.push_back(*it);
    }   
  }

 private:
  enum { INT, VECTOR } type_;
  int val_int_;
  std::vector<NestedInteger> val_vec_;
};

int main() {
  NestedInteger ni1{1};
  NestedInteger ni2{1, 2, 3}; 
  NestedInteger ni3{{1}, 2, 3}; 
  NestedInteger ni4{{1, 2, 3}};
  NestedInteger ni5{{1, 2, 3}, {4, 5, 6, 7}};
  return 0;
}

And I got compilation error:

t.cpp:29:44: error: no matching function for call to ‘NestedInteger::NestedInteger(<brace-enclosed initializer list>)’
   29 |   NestedInteger ni5{{1, 2, 3}, {4, 5, 6, 7}};
      |                                            ^
t.cpp:12:3: note: candidate: ‘template<class T> NestedInteger::NestedInteger(std::initializer_list<_Tp>)’
   12 |   NestedInteger(std::initializer_list<T> ni) {
      |   ^~~~~~~~~~~~~
t.cpp:12:3: note:   template argument deduction/substitution failed:
t.cpp:29:44: note:   candidate expects 1 argument, 2 provided
   29 |   NestedInteger ni5{{1, 2, 3}, {4, 5, 6, 7}};
...

I read the list initialization in cppreference here. And I can tell the expression belongs to the first syntax type:

T object { arg1, arg2, ... }; (1)

initialization of a named variable with a braced-init-list (that is, a possibly empty brace-enclosed list of expressions or nested braced-init-lists)

I can't understand the explanation about the effects.

Could you help to figure out:

1. Which item in the explanation does this case match?

2. Is this a non-deduced contexts described here?

By the way, if I change the template to specific type as the commented line, the compilation error disapeared.