Isn't passphrase something like a password but with symbols? Isn't the right term Seed recovery phrase, seed phrase or Backup Phrase?
Yes, it's basically seed phrases for your wallet.
OP doesn't need to worry about privacy with the
https://iancoleman.io/bip39/ tool. He can use it while offline and till be able to generate the private key he requires.
I was referring to generating signed message online with the help of private keys generated from
https://iancoleman.io/bip39/, which doesn't seems to be a safe method due to the requirement of putting your private keys online.