There has been a sort of agreement in which Marcos supporters effectively supported Duterte last election in return for Duterte supporters supporting Marcos in this election.

So, a lot of his supporters, yes. Marcos is now at the head of what is essentially a pro-Duterte party. However, Duterte himself basically claimed that Marcos would make a weak President. I don't believe he had ever officially endorsed Marcos even after it was clear that he had no successor to his legacy because of the bungling of two people: his daughter, and close ally Bong Go.

Because of his daughter's hesitation to run for the presidency until doing a partial U-Turn and deciding to run for VP, Duterte wanted a close ally who he has known for decades, Bong Go, to replace him. However, Bong Go also had limited motivation and didn't announce his candidacy until it was far too late, by which time much of the Duterte camp had decided upon Marcos. Bong Go then withdrew from the race, particularly because of poor polling and because he would only divide the Duterte supporters between himself and Marcos at that stage, potentially leading neither to win because of the Philippines' first-past-the-post electoral system (there is no Presidential runoff like in France).

Effectively, the reluctance of both people allowed the establishment to retake the Presidency.