Except for the German federal prosecutors, Dutch officials, former Ukranian commander in chief, and the sabotage team themselves, I suppose you’re right.
Days after the attack, in October 2022, Germany’s foreign secret service received a second tipoff about the Ukrainian plot from the CIA, which again passed on a report by the Dutch military intelligence agency MIVD. It offered a detailed account of the attack, including the type of boat used and the possible route taken by the crew, according to German and Dutch officials.
Inasumuch as you can have a source for a covert operation after the fact.
“Oops” ¯\_(ツ)_/¯