We see later in the series that the nation of Marley have been turning people into titans and dumping them on the shore of Paradis. But why? King Fritz warned the world that if the citizens of the wall were ever threatened by an outside force he would send the colossus titans inside the walls to flatten the Earth. So why send titans? I assume its a kind of revenge for the great titan war, but why risk the wrath of the king?
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 "Declaration of War" 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 " one gazillions years ago their ancestor killed your ancestor so turn them into zombies and lock them behind bars").
So because high-rank people knew about the bluff and hated Eldians for their "sins" 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.