One would be unwise to rely too heavily on the reporting (transfer journos are out of their depth on this stuff), but this is not what has been reported.
Forest HASN'T formally registered a complaint with the league, they have threatened legal action in letters to Spurs and MGW's agents, alleging that the bid they received could only have been made as a result of breaching the confidentiality of the contract (the claim against MGW), and that the breached information could only have been acquired via an approach to the player without permission in violation of PL rules (the claim against Spurs).
The message is not "Premier League, come tell these guys they've been bad", it's "we will not honor this clause, whose execution represents an unlawful conspiracy against us".
They aren't asking, they're telling.
This is a breach of MGW's contract, and MGW has responded by doing nothing. Why? Search me. Seems like an admission of guilt, but who knows.