I don't get what alternative explanation is there?
Ter Stegen did not deny signing off to presenting the medical reports? It's fake news?
The only ridiculous thing would be if it is true in fact that he will be back in less than 4 months, but in that case, there would be no secrecy?
Such report hurt him, if he indeed sees himself being able to return before 4 months.
The club won't allow it to happen, as per La Liga rule, they will get a hefty fine (3 times the amount the registered player through exception, so 240% of half of his salary).
There is also a question of trust, especially with the smear campaign ran against him, with the likes of Polo accusing him of skipping training because he is mad at the club, while he was injured. Then trust to the medical team, as he had to repeat almost every operation he had under the supervision of the club.