It appears that if you restrict changes under the Screen Time options on any iOS device on an account, it will restrict those changes on any other iOS or MacOS device on that account. This capability was apparently driven by Parental Controls considerations and designed to prevent a child from changing iCloud account parameters that would allow them to remove the controls from any device on which they have an account. Many of us use this feature to prevent access to a stolen iPhone with a stolen pin that would allow changing iCloud account parameters without an iCloud password. It would be nice, but is not possible now, to be able to lock down an iPhone without also locking down an iPad and a Mac. The Mac getting locked is particularly problematical because the alternate Screen Time PIN access is not available on MacOS.