Why did Marley send titans to Paradis Island in the first place?
High-ranking people in Marley nation such as the Tybur family probably knew about the king's lie. As Willy Tybur admitted knowing this in the episode &quot;Declaration of War&quot; episode 4 of the final season.

Also, the beast titan (the predecessor of Zeke) knew about the oath. I don't remember the manga chapter.

However (perhaps) hiding this secret was used as a tactical strategy to motivate people to attack the island (not everyone is going to buy that &quot; one gazillions years ago their ancestor killed your ancestor so turn them into zombies and lock them behind bars&quot;).
So because high-rank people knew about the bluff and hated Eldians for their &quot;sins&quot; and they wanted their resources and the founding titan, they keep harassing the island while used that bluff to motivate people against them at the same time.