That approach may do so well because many speech repairs are very short, involving only one or two words Shriberg and Stolcke (1998), so the reparandum, interregnum and repair are all contained in the surrounding word window used as features by the classifier.
the probabilistic model of repairs explored here seems to be most successful in identifying long repairs in which the reparandum and repair are similar enough to be unlikely to have been generated independently.