The mount command with the space will work. It is asking to mount the (nonsensical) device "rw" on directory "/", but because we also specified the "remount" option, the given device is ignored and the existing device is reused. The "rw" option is actually the kernel default, so omitting it does not matter.
The mount command without the space fails in recent images. /etc/fstab specifies the devices by partition UUID, and even though the device will ultimately be ignored for this command, mount tries to determine the device name from the UUID first. This fails when we have booted in single-process mode, presumably because of the lack of /proc, /sys, udevd, systemd, or something like that.
We should probably recommend the command as: