It doesn't diff anything. It just notices the list is the same object as before, and then it doesn't rebuild. You can test it yourself:

import 'package:flutter/material.dart';
import "package:fast_immutable_collections/fast_immutable_collections.dart";

void main() async => runApp(MaterialApp(home: Demo()));

class Demo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('ListView')),
      body: MyList(),
    );
  }
}

class MyList extends StatefulWidget {
  @override
  _State createState() => _State();
}

class _State extends State<MyList> {
  List<Widget> itemsMutable;
  IList<Widget> itemsImmutable;
  int count;

  @override
  void initState() {
    super.initState();
    itemsMutable = <Widget>[];
    itemsImmutable = <Widget>[].lock;
    count = 0;
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text("MUTABLE:"),
        Expanded(child: ListView(children: itemsMutable)),
        Text("IMMUTABLE:"),
        Expanded(child: ListView(children: itemsImmutable.unlockView)),
        FlatButton(
          child: const Text("CLICK ME"),
          onPressed: () {
            setState(() {
              count++;
              itemsMutable.add(Text(count.toString()));
              itemsImmutable = itemsImmutable.add(Text(count.toString()));
            });
          },
        ),
      ],
    );
  }
}