I think that makes sense, although I might think Lurline would be more likely than Ozma to be the one who hid the entire continent. It's one thing for Ozma and Glinda to make their own country invisible, but I don't think they really have any say over the surrounding lands (well, except the Nome Kingdom, I guess).

There was an Oziana story in the eighties that had a military expedition accidentally finding the Ozian continent using radar, even though it was invisible to the naked eye.