How to Change OS on My Devices
The operating system listed on my iMac in "My Devices" is wrong.
How do I update it?
iMac 27″, macOS 11.6
That is a boilerplate notice. You did not extend the red outline in your screenshot to the very important words "or later." The point of the message is that you if have any devices running an operating system earlier than those listed, those devices will not be included in the list below the message even if they are signed in.