I don't. It's based off my own testing, not by disabling SMT, but by running either <core_count> or <thread_count> parallel threads. It was my own code, so it's possible code that uses SIMD more heavily will see a less-significant speed-up. It's also possible I just measured wrong; running Cargo on a directory with -j16 and -j32 takes 58 and 48 seconds respectively.

>From a hardware perspective, I can't imagine how it could be physically possible to double performance by enabling SMT.

It depends on which parts of the processor your code uses. SMT works by duplicating some but not all the components of each core, so a single core can work on multiple independent uops simultaneously. I don't know the specifics, but I can imagine ALU-type code (jumps, calls, movs, etc.) benefits more from SMT than very math-heavy code. That would explain why rustc saw a greater speedup than Cinebench, as compiler code is very twisty with not a lot of math.