notbRay tracing is a light propagation model that treats light as rays (a stream of photons). So the algo itself is single-threaded and built around relatively simple operations.
Now, rendering is a task of running a ray tracing method many times independently. These runs can be done in parallel, as you said.

But the original issue was about poor HT effectiveness in this benchmark. I won't really comment on whether it's actually that bad (there's not enough data yet). But I wouldn't be surprised.
Remember how HT (SMT) works. A core in your CPU can hold and execute 2 processes at the same time... up to a point when they need a core element that's singular (e.g. ALU).
Some tasks leave a lot of computation potential unused (because the CPU spends more time on just working with data and communicating outside).
Ray tracing is very intense computationally.
I've seen a few tests showing that rendering doesn't benefit from HT as much as some other applications do.
Here's an example (quite extreme, as the site name suggests):
www.extremetech.com/computing/133121-maximized-performance-comparing-the-effects-of-hyper-threading-software-updates
You should be able to find a more recent one to check if things improved drastically since 2012 (but they haven't ;-)).

WavetrexSo we live in the day where absolutely free software is subjected to copyright and take-down threats.

Okay. I'm done.
Unplugging my PC's, throwing them out of the window.
No more software, no more hardware, no more internet for me.

Time to dust off that Stradivarius... and restart practicing on it.