Thank you very much @theymos for the reply. I just remember that I use an e-mail that I plan to create uniquely for my bitcointalk but I forget to do so like many years ago.
I will keep the thread open for a while for anyone who has anything to say on this update. I already updated the OP to include @theymos official response and mark the title as solve to help other user informed in case they encounter same confusion like what I did. Thanks guys!
I also happen to have created my own account with an invalid email as of the time I registered it, but after I started being active here, I saw a need to register that email and make it valid. This is because there might be a need for that email in the future, which I can never tell for now.
This unexisting email that is used to create a Bitcointalk account is, I believe, very popular among members here since the forum doesn't request email confirmation before they proceed with the registration.
I believe such a method might be for privacy reasons, and the forum also doesn't send news letters to members using the email. If it's becoming a thing of concern, I will suggest that for new member registration, email confirmation before proceeding be mandatory to confirm if the email is valid.