I don't think it's a fact - this article[1] explains a lot of the complications around ownership. Currently it's taxed as part of the UKCS (UK Continental Shelf) and assignments of operation are part of the North Sea Transition Authority (NSTA)[2] which doesn't consider any particular region of the UK as owners, but as part of a whole regional area instead.
[1]: https://www.bbc.co.uk/news/uk-scotland-scotland-politics-200... [2]: https://www.nstauthority.co.uk/