I dont know why Raspbian has Stretch packages available on the archive.raspbian.org and the mirrordirector.raspbian.org location.
But why not copy the whole (default) line from my sources config file including the "rpi" packages ?
deb http://mirrordirector.raspbian.org/raspbian/ stretch main contrib non-free rpi
EDIT: Weird thing is, they even did not bother to deploy new way of apt (Version>=1.1) selecting mirrors:
$ host -t srv _http._tcp.deb.debian.org
_http._tcp.deb.debian.org has SRV record 10 1 80 prod.debian.map.fastly.net.
$ host -t srv _http._tcp.apt.xbian.org
_http._tcp.apt.xbian.org has SRV record 10 1 80 xbian-apt.mirrorservice.org.
_http._tcp.apt.xbian.org has SRV record 10 1 80 apt.xbian.org.
_http._tcp.apt.xbian.org has SRV record 10 1 80 apt1.xbian.org.
_http._tcp.apt.xbian.org has SRV record 10 1 80 apt2.xbian.org.
$ host -t srv _http._tcp.archive.raspbian.org
Host _http._tcp.archive.raspbian.org not found: 3(NXDOMAIN)
$ host -t srv _http._tcp.mirrordirector.raspbian.org
Host _http._tcp.mirrordirector.raspbian.org not found: 3(NXDOMAIN)