iCloud email hiding generates addresses on iCloud domains, i.e. services will begin to flag them as a commonly-used disposable address provider and disallow them.
Also completely worthless to the vast majority of people who are not on Apple devices.
Also also, 1Password's integration with the email isn't managed by them. They talk to Fastmail, Fastmail spits out an address and tells it to 1Password, who then fills the form with it. I can ditch 1Password at any time, even delete my account, and lose nothing.
https://support.apple.com/en-gb/HT210425