The setting "All apps can ask to sign me in" being greyed out for your primary account indicates that this is a system-managed setting that cannot be changed by the user. In Windows 11, certain sign-in options are configured based on the account type and the policies applied to it.
When this setting is enabled, it means that apps will have the ability to prompt you to sign in, but it does not necessarily mean that you will be asked to sign in every time you use them. If you have already logged into certain apps with your primary account, they may remember your credentials and keep you signed in automatically. However, if you have not logged into an app yet, or if the app requires re-authentication, you may be prompted to sign in again.
In summary, while the setting is mandatory and cannot be changed, it does not imply that you will always have to sign in to every app every time you use them. It allows apps to ask for sign-in when needed, based on their individual authentication requirements.