Not exactly. Their designed advance width is 1120 "font units". The TrueType convention (which I followed) is 2048 font units per em. So they're close but no

Not exactly. Their designed advance width is 1120 "font units". The TrueType convention (which I followed) is 2048 font units per em. So they're close but not quite 1/2 em wide. However, many rasterizer round the widths to whole pixel sizes. Taking that into account, I do see them coming out to exactly half the width at certain sizes on this machine.