Jon Skeet wrote:

If the time in UTC is known, but the offset to local time is unknown,
this can be represented with an offset of "-00:00". This differs
semantically from an offset of "Z" or "+00:00", which imply that UTC
is the preferred reference point for the specified time.

That's not what we intend to convey in this case, is it?

No, it's pretty much what we want to convey. In the case in question, the time in UTC is known, and local time is undefined which means it is unknown. An example is Antarctica/Troll before 2005, when Troll Station was uninhabited