I'm posting my understanding of the article as it pertains to the question here. It seems that the screen grid which comes first in the path of the positive ions is has higher potential compared to the accelerator grid which comes after the screen grid. This generates an electric field pointing towards the end of the thruster and the positive ions will be accelerated out of the exit. If I understand correctly, you're asking how come the positive ions passing the accelerator grid don't slow down and come back towards the accelerator grid since it has a negative potential. If that is your question, the following excerpt from the same source gives the answer:
The positively charged ions are accelerated out of the thruster as an ion beam, which produces thrust. The neutralizer, another hollow cathode, expels an equal amount of electrons to make the total charge of the exhaust beam neutral. Without a neutralizer, the spacecraft would build up a negative charge and eventually ions would be drawn back to the spacecraft, reducing thrust and causing spacecraft erosion.
As mentioned in the article, the electron beam and positive ion beam neutralize each other after the accelerator grid such that the created electric field won't affect neutral particles. Please let me know if this answers your question or if you have any other questions.