The reason the TB image is not listed in the USB/SD Creator app is we need to modify the app to not crash when RK images with suffixes are listed in the JSON data. Fixing that is somewhere on our to-do list, but not yet at the top of the list.
NB: All of our RK images are currently in a persistent 'alpha' state while we finish reworking the codebase around the mainline kernel .. the alpha's use the older RK 4.4 kernel. That work is approaching the point where we can switch and then start the journey towards LE10 which is when things should be officially official.