Well, there is nothing too-complex to understand here. This mount looks like a duplicate, but it really isn't.
What OpenWrt intends to do is to mount a subdirectory of the filesystem stored on /dev/mmcblk0p1 to /boot. This is impossible to make in one go. So, first, they mount the whole filesystem from /dev/mmcblk0p1 to /boot. Then they have access to the subdirectory, but it is at /boot/boot, which is not what they want. Thus they use a second mount (a so-called "bind mount", a special mode which makes an already-existing directory appear elsewhere, instead of taking a block device) to make it appear directly on /boot. However, the Linux kernel displays the underlying block device for such bind mounts, which is what confused you.