Yup, men Olympic style archers need to reach out to 90 meters in a FITA round. To do this he needs a lower under chin anchor. Split finger facilitates this much better as a three under would create interference on the chin. Split also gets the arrow just a bit lower on the string then the three under.
Keep in mind that an Olympic style archer must have an open aperture or pin (no magnification), at 90 meters, there is literally in many cases just millimeters clearance between the aperture and the arrow passing by (that is if he wants to aim directly at the intended target).
Another thing not mentioned is that the Olympic archer also curls the index finger upward around the string (if using NTS) which could also cause problems when shooting 3 under.