Select All checkboxes by default in inner mat table not working

I have a nested mat table with expandable rows . The inner table has checkbox and by default all the checkbox has to be checked in the nested tables on load. Here is the stackblitz with issue . Can someone help to resolve

https://stackblitz.com/edit/angular-nested-mat-table-triplenested-iavs62?file=app%2Ftable-expandable-rows-example.ts