Does this new passwords app also hold the passwords for any locked .pages documents?

No. When you place a password on a Pages document, it asks if you want to remember that password in Keychain. Still true with Pages v14.2 on macOS Sequoia v15.0. So new or past passwords on Pages documents are not migrated to Passwords application.

See the differences between the Passwords application and Keychain:

Passwords User Guide - Apple Support

Keychain Access User Guide for Mac - Apple Support

When you place a password on a Pages document, and you select the option to have Keychain remember that password, later, when you attempt to open that document with Pages (on the same Mac, your account), it asks Keychain if it has the password for that document. If so, the document just opens normally.

If you place that password-protected Pages document on an iCloud drive, or a USB stick and then attempt to open it with a different Mac, that Keychain won't have the previously stored password, and your unlock effort is rejected. Then the wisdom of Yellowbox kicks in… 😉

I just performed a test. I password-protected a Pages document, remembering in Keychain on macOS Sequoia v15.0 and using Pages v14.2. I then copied that document to my iCloud Drive. This was on my M2 Mac Mini Pro.

Next, I opened my 2021 16-in MacBook Pro M1 Pro running Ventura 13.7 and Pages v14.1. Same account name and password as on the Mac Mini Pro. I located the password-protected Pages document on iCloud Drive and when I attempted to open it from my user account, it threw a password box. Even with Touch ID enabled in that password dialog box, it would not open the document. Otherwise, Touch ID is working on the MacBook Pro. The same result was when I copied the Pages document to the local drive on the MacBook Pro.

This led me to verify (as true) that passwords and keychains were being synchronized on both Macs and this was enabled on both and after 30 minutes of waiting, I still could not automatically open that password-protected Pages document on macOS Ventura.

So the wisdom of Yellowbox in this case is:

The good news: passwords work.

The bad news: passwords work.

The good news: passwords work.

The bad news: passwords work.

Regards,

Ian.