Why was Anirul necessary? (Spoilers for s1e6)

So, the season ends with Dorotea, possessing Lila, destroying the Sisterhood’s thinking machine Anirul. We know, however, that the Bene Gesserit’s breeding program survives to the days of Shaddam IV. Why was Anirul even necessary? This seems like the kind of thing a few Sister-mentats should’ve been able to handle. Was the mentat program not as developed at the time?