Ok, now I see the confusion is different.
The instructions to download a wallet backup / export the account passphrase from the myNXT web wallet always refer to steps to follow in the myNXT web wallet,
https://wallet.mynxt.info. Not to the official Nxt client, the one you can download from
https://nxtplatform.org/download.
These steps before myNXT closes (and again, myNXT is a third party service and unrelated to the Nxt core development, so this is just a third party tool closing, nothing else) are one of the two options for myNXT users (see their
FAQ) to either transfer tokens to another Nxt account managed with the official Nxt client, or obtain the passphrase for the Nxt account they managed with the myNXT web wallet.
All Nxt accounts are unlocked by a passphrase, equivalent to the private key in other platforms like Bitcoin or Ethereum. When you create an account using the Nxt Client, you are reminded to save the 12-word passphrase that the client generates. This passphrase is necessary to send any transactions from the account. But the myNXT web wallet, for design and security reasons, does not work directly with that passphrase, but using a separate "master password" to encrypt the user data, including the passphrase for their Nxt account, in the site database. So any user who wants to have the actual passphrase, to use that same account with the Nxt Client, needs to download it from myNXT and decrypt it, as the instructions say.
Since you say you already had generated a passphrase + account using the official Nxt Client, you have the two options in the FAQ linked above. You can transfer your assets + NXT from your myNXT web wallet to that account you already have the passphrase for, or you can download a wallet backup from myNXT (
https://wallet.mynxt.info, settings > security from the dashboard) and then use the passphrase obtained to access the same account in the Nxt Client.