Yes it is downloading 15GB and also some "useless stuff for other devices".
But this useless stuff is very small, maybe 50MB.

Your next step will be to find the device and kernel sources for your device and place them in the appropriate folders.
You also have to either download the vendor blobs or extract them from your phone.

It also helps to have the cyanogenmod sources for your device as reference. (another 30GB)

After these steps you can try to build Ubuntu touch for your device. There will be a lot of errors and you will have to solve them one by one

Please take a look at the Ubuntu porting guide: https://developer.ubuntu.com/en/start/ubuntu-for-devices/porting-new-device/#anchor3
Your device/huawei/angler/BoardConfig.mk will have a variable TARGET_KERNEL_SOURCE which has to point to your kernel sources.
The guide is a bit unspecific about the vendor blobs so i suggest to read the following: http://wiki.cyanogenmod.org/w/Build_for_angler#Extract_proprietary_blobs

Good luck

One problem you will run into is, that the vendor blobs that you have to extract from your phone are designed for android 6.0.0 or 6.0.1 while the source you downloaded with phablet-dev-bootstrap is based on android 4.something

I heard chances are high that this will not work together