wondersnickers

Cinebench is not the perfect tool to test throttling for enclosed systems like this. In a real world scenario the GPU is likely to generate additional heat, the shared thermal solution will negative influence throttling by adding a massive heat source.

100°C is also likely kill components in this machine over time. A lot of components in a mac are not designed to withstand that amount of heat. One example: More heat / heat fluctuations can increase humidity:  All new Macs have the 50volt line next to a vital low voltage line. This can fry your system, apple will blame you for water damage and charge massively for repairs, which will reoccur in the next "repaired" system. It's a flawed design stay away if you want to render a lot.

PS: a similar post of mine some months ago to the iMac "disappeared".

"It never dipped below the advertised frequency":
Your Cinebench screenshot shows this iMac cannot thermally sustain the 9900k's normal boost clock and is already maxing out close the base clock at 85 degrees Celsius, even without load on the GPU, in a enclosed design.
In your benchmark the 9900k runs just at 3800mhz, that's only 200mhz above baseclock (3.6), this about 1 Ghz short to the 9900k's intended use:
The 9900k's Max Turbo Frequency is usually: 2 cores on 5 ghz, 2 cores on 4.8, 4 cores on 4.7-> https://en.wikichip.org/wiki/intel/core_i9/i9-9900k ;
The Base Clock is 3.6 ghz. 

-> So the 9900k's boostclock can only actually be utilized for short bursts of time. The Boostclock is where Intel CPU's shine! You are basically paying extra to have a stronger Cpu on paper, but in real life.
-> Cinebench R20 doesn't stress the GPU. The iMac is an enclosed design, which means that the Vega GPU will generate a lot of additional heat in a lot of real world scenarios, this heat is shared and is highly likely to do additional thermal throttling, which is not shown in your tests! 

Having a machine under that much heat all the time is not good for its longevity. Especially when there is a lot of glue in use, heatspikes, humidity, chances for failure increase. Don't forget: This things are very expensive to service! 

The iMacs design, in therms of an High End Device is "design before functionality". Why do not change the iMacs design to have far less of a heat problem and therefor much more computing power? FE: You could put massive Heatsinks on its backside, it could look very cool and perform waaaaaaay better, plus potentially live longer.