What, according to Borges, is the feat never achieved by Poe, but amply achieved by Chesterton?

Chesterton's endings were never predictable, as Poe's often were.

Chesterton used a newspaper article about a murder as the basis of a story $-$ and ended up correctly solving the murder.

Chesterton created a detective character, Dupin, who served as a model for countless detective characters that followed; Poe's detective characters proved forgettable in the end.

Chesterton wrote stories at once fantastic and, in the end, having a detective solution.