A Reddit user has managed to find out the reason for the high throttling seen in the 2018 Apple MacBook Pros with the Intel Core i9 CPU and it turns out that it is due to power throttling of the VRM than thermal throttling of the CPU. The user has managed to manually alter the turbo power limit for the VRM and thus, was able to achieve steady turbo boost without throttling.
https://www.notebookcheck.net/Throttling-in-the-2018-Core-i9-Apple-MacBook-Pro-traced-to-improperly-set-VRM-power-limits.318176.0.html