I think the issue is that an Apple ID is used for multiple things (iCloud backup, the app store, etc.), and also that the seamlessness of SMS/iMessage (usually a good thing) causes them to be conflated.

This person doesn't use their Apple ID for communication, because they have an Android phone. They probably purchased an iPhone for their child and set up the Apple ID using their email address. So to clarify, they weren't using their Apple ID with multiple devices, they just set up their child's Apple ID using their email address.

I'm not trying to slam Apple here, it's a complicated issue. I'm just saying that through a sequence of pretty reasonable steps we ended up in a situation where I was texting a colleague's daughter. Fortunately they were about parking my car, but it was pretty weird regardless.

edit: I guess they may use their Apple ID on their laptop as well, so it is on multiple devices. I just don't think they use it for iMessage.