Where the sidewalk is continuously adjacent to the carriageway (no big grass areas etc just an immediate kerb), and has no special access (eg shared cycleway), then the extra footway is a duplicate way.  The public highway is mapped and carries the foot traffic.   We don't add a second footway beside a farm track if it has a public footpath, we add foot= and designation= to the way.  This is just the same; a duplicate way offering no choice of route but adding massively to the complexity of the foot routing calculations and the maintenance of the map.  The path here is a massive GPS routing trap, which says that if you route to or from anywhere in the block around Walmer Gardens (gardens) then you MUST go via the E connection of Walmer Gardens (road) and Erlesmere Gardens (road).  This is fiction, bad mapping, bad routing.