The “Basic Quizlet” note type is based on “Basic (optional reversed card)” note type and contains the empty “Add Reverse” field that can be used to selectively add reversed cards.
if you additionally enter something in the “Add Reverse” field (like a ‘y’), Anki will generate a reverse card as well
https://docs.ankiweb.net/#/templates/generation?id=reverse-cards
Select all the cards in the subdeck with the card browser - Notes - Find and Replace (Ctrl+Alt+F).
Find:
Replace: y
In: Add Reverse
OK