Amdahl's law is one thing, and an inefficient implementation of a concurrent program is another thing.

We do not know how much of the bad scaling of Geekbench 6 was due to Amdahl's law and how much was caused by an inept implementation.

Moreover, Amdahl's law is really important only for very short computational tasks, i.e. which do not need more than a few seconds or minutes for completion.

The bigger is the job that must be accomplished, the more likely it is that it can be reorganized in a way that tends to be embarassingly parallel, so that Amdahl's law has little importance.

So for jobs as big that they need hours, days or weeks to be completed by a computer, if they are well planned it is frequent to reach scaling factors not much lower than the number of available cores, even if that may be much more difficult to achieve for short jobs.