I'm not sure that page works with the old mnemonic lists though, it returns 'unknown word' for one of my mnemonic words that is certainly in this list: https://web.archive.org/web/20180406224820/https://blockchain.info/Resources/js/mnemonic/mnemonic_words_v3.txt
Here: login.blockchain.com/legacy-pages/js/mnemonic/mnemonic_words_v3.txt
Or here: github.com/blockchain/blockchain-wallet-v4-frontend/blob/development/legacy-pages/js/mnemonic/mnemonic_words_v3.txt
I don't know if that's actually used though since I never saw an account recovery phrase with words: "a", "aa" "aaa" and "aaaa".
Probably used as prototype or something.
But maybe you're referring to a different word list.
See if your words belong here instead: github.com/blockchain/blockchain-wallet-v4-frontend/blob/development/legacy-pages/js/mnemonic/mnemonic.js#L488
(all those words are in that 65,591 words list)
Anyways, since you got the important details and login credentials, just go for the "change email" method like they did in the other thread.
Trying to get to the old mnemonic wont get you anywhere since it'll just show the login info that you already have.