Just looked at the Intel's technical documentation. They say that 45W is enough to cool down the i9. Furthermore, from Intels 8-th gen data sheet Volume 1 (Table 5-1) defines TDP as following:
Sounds quite clear to me: when running "high-complexity workloads", 45Watt of cooling should be sufficient to maintain the base frequency. And MBP's cooling is good enough for that, as shown by the performance with earlier gen CPUs. Yet it doesn't for some reason. Also, there is no mention of 100W or anything like that anywhere in the documentation. And yeah, Apple should have tested nada, nada. Of course they did test. But don't you think its strange that the technical documentation of a SKU and its actual thermal performance are so different?
The i9 is designed to replace earlier top-end CPUs such as i7-6920HQ/7920HQ etc. With Coffee Lake, Intel does not offer a high-end i7 CPU anymore at all. And their top-end is as it seems clocked waaay too aggressively for the silicon to handle it.
My point is: either Intel is being a bit dishonest in their technical documentation, or we have some other issues that are not yet properly studied. Of course Apple carries responsibility. But I don't think its reasonable to tell that they shouldn't have used a chip that is marketed and described by its manufacturer as being suitable for their laptops.