This passage is accepted as a statement of gathering. After the Lord's redemption of Israel (or so it is structured in this passage) the gathering of scattered Israel will be effected. Along these mountain "ways" and along these exalted highways will travel the dispersed of the covenant people. It is not surprising to have them returning from the "north" as that is the direction typically ascribed to the dispersion (with some historical veracity, although it appears that "north" could also have a symbolic function in Old Testament texts). They are also gathered from the west, and from the enigmatic Sinim.
"The gathering promised in verse 12 will be so extensive that the remnants will come from as far as the land of Sinim. There has been much debate upon the location of Sinim. Some scholars believe it to be a place near Aswan, Egypt.... Others feel that the deserts of Sinim in Egypt form no suitable contrast to the gathering from the north and the sea as promised in the earlier part of the verse, since Egypt is too close to the Holy Land.... Whether Sinim is near or far from Jerusalem, the fact that Israelites gather from there seems to be an unexpected miracle." (Ludlow, 1982, p. 411-412).