Code density is not all about instruction length. It's also about instruction count. With shorter instructions you usually need to use more instructions to do the same thing (e.g. extra MOVs or stack spill etc).
Does the footprint of Thumb (including total bytes, count of opcodes, speed, likely other things that I have not thought of) impact the conclusions in the paper?
Interesting that Thumb has been removed from AArch64, and that Intel never added anything like it.
Thumb was a thing for embedded systems with very limited RAM etc. It was not designed for optimal speed.
My guess is that ARM64 targeted higher end, and that severe memory constraints were no longer considered a big issue. ARM has its Cortex M4 (Thumb only microcontrollers) and the likes for those markets.