That reads quite different to what was claimed though.Did you not see Forest report Spurs about it? Secret is exactly what it was…
“Forest's bosses shut down lines of communication with Spurs, accusing them of an illegal approach for the player and a breach of the confidentiality clause in his contract, which prevented any public release of the value of his buy-out clause.
Forest have made it clear in letters to both Tottenham and Gibbs-White's agent that they reserve the right to pursue legal action in the future, and while Spurs remain interested in buying him and are prepared to meet his £60m clause, they have not yet pursued the matter.”
That suggests the clause itself is not secret, in the sense agents and clubs can't know about it, but rather there is an NDA on it from the press once activated.