OpenAI didn't have access to every poster's email when they crawled reddit. If you're making posts or have an account name that are easily tied back to your personal identity, that's on you. But you could make an account with any random username you wanted, that keeps you anonymous as far as OpenAI is concerned.
My point was only that 17 years ago - and for more than a decade, reddit required no email address as a requisite to create an account... so it was truly anon... then they tie all (new) accounts to emails now - which makes it a trivial click for survelleince to ID your reddit account...