Nope. It's just for Wii data.
No that is the SD card port, they were referring to plugging in a USB SD card reader which is essentially the same as a USB stick.
That should work but it might not be as reliable as a USB stick which should have their own wear leveling. Also as you point out, a HDD is a LOT cheaper per GB so the only point really is if you already have some SD cards to use or know the can reuse the one you bought for the Wii U later if you upgrade.