Pretty much. Booting the phone up while Vol+ button is held puts it into USB download mode, at which point you can load the bootloader from a PC over the USB port. uuu is used to do that (boot u-boot), and also to communicate with that loaded u-boot instance so you can flash things or load them into memory and boot from there. That’s what Jumpdrive’s uuu script does - it loads u-boot, then uses it to load a kernel and initramfs into RAM and tells it to boot it from there.
Once Jumpdrive boots (which is pretty much just Linux with a small initramfs that contains Busybox and bunch of scripts), it displays the splash screen, exports eMMC and SD card over USB as mass storage devices, sets up serial console and Ethernet (also over USB) and runs a telnet daemon.
Technically, you don’t need to do the battery dance, but the instructions tell you to do so because it increases the chances of it working in some edge cases, like battery being flat, or SoC being stuck in some weird state. This helps because of the LED feedback you get when there’s no battery in - so you can match the LED output you see with the instructions and know that things are going well, there’s enough power available for phone to actually run etc. Without that, there’s no indication of progress, as the only feedback USB download mode gives that informs you it actually booted into USB download can only be seen over USB, so if you don’t see it on your PC you’re pretty much in the dark and figuring out why it’s not working is mostly trail-and-error. With these slightly more annoying instructions, we can at least weed some common failure cases out early 
(that said, I never take the battery out to put my phone into flashing mode - once you’re familiar with the device enough to recognize which state it’s in by yourself there’s not much reason to do so)