Definitely sounds like thermal throttling to me, since I've experienced just what you described multiple times with my 2011 and 2014 MacBook Pro's. Don't know if you still have that 16" Intel MBP, but if you do the following might be helpful:
What's different with what's happening to with you vs. a normal kernel task is this:
1) Its percentage is extremely high.
2) Your computer has become essentially unresponsive. That doesn't happen normally even if you are running a set of processes that are using all the cores (e.g., run a program in each of x different instances in Terminal, where x = number of cores). In the latter case, the OS is smart enough to bump down the priority of those other programs when you actually need to interact with your computer. It will be a bit more sluggish than usual, but it won't be unresponsive like what you experience with thermal throttling. By contrast, with thermal throttling, the OS does the opposite
because the point of the kernel task is to keep you from using the CPU much at all.
One simply way to test for thermal throttling is to point a desktop fan on high at the back of the MBP and see if it goes away, and then remove the fan and see if it comes back; repeat a couple of times. This is a sufficient but not necessary test for thermal throttling; i.e., if that happens consistently, you've got throttling; but if it doesn't, the test is inconclusive—you might still have throttling, but the fan isn't sufficient to make it go away.
More details on what I experienced: In both laptops, the OS would create a phantom high-load kernel task. I believe this didn't actually use the CPU, and that its purpose was to instead simply block off capacity so it couldn't be used by real processes, i.e., that it directed the scheduler to act as if most of the processing capacity was unavailable. Here's a screenshot from the 2014 MBP when it was happening:
An additional direct indicator of thermal throttling, at least in Intel machines, is that the CPU speed was slowed to 800 MHz (screenshot taken during same throttling event as above):
Thus between blocking off most of the processing cores (I think), and slowing the speeds of all cores to 800 MHz, all real processing on the CPU was slowed to a crawl (e.g., ~30 sec between when I pressed the keys to call up the force quit menu and when it opened).
My laptops were always connected to an external monitor, so the dGPU was always running, and my guess is that the throttling was due to dGPU overheating—especially since the CPU temperature wasn't very high; plus disconnecting the external monitor, which would cause the system to switch to the integrated GPU, would eliminate the throttling after about a minute. [Or I could keep the monitor connected but, as I mentioned above, aim a desk fan on high at the back of the laptop; this would also eliminate the throttling after about a minute.]
I forget the repair history with the 2011, but with the 2014 the problem appeared a couple of years after purchase, and Apple replaced the motherboard, but the throttling returned after a year. My guess is that there was a problem with the stability or application of the thermal paste. After the 2nd replacement, I had no further problems, so I guess Apple (or their repair center) found a better quality thermal paste (or a better way to apply it).
