Essentially, you just have been arguing over what is the primary source of heat

.
I've been arguing that running a core at 100% load does not constitute a light load and its fairly normal for a laptop to run hot during such an operation.
Expecting companies to throttle their apps to reduce heat dissipation is probably not realistic.
It's not about throttling the app, it's about efficient use of resources. An app probably shouldn't ask for 100% CPU uptime if it doesn't do any useful work. Especially with modern hardware that can go from zero to crazy in milliseconds. Apple has been making this point over and over at WWDC for the last couple of years, and they have introduced many technologies to that end.
Nothing in this discussion is exclusive to 2018 machines by the way. It has been the case since turbo boost is a thing.And for some reason neither my 2018 i9, not the four other 2018 machines (one 15" and three 13") are running abnormally hot under regular operations.
By the way, just to be sure, I now started copying a 12GB file from the laptop to a NAS. First, some validation routine started (100% single thread CPU utilisation, 30W power draw, max turbo boost), so the temps obviously shot up to around 80, but around 30 seconds later, the actual copy process has started — the CPU utilisation is now at under 5%, 5-7 watt power draw, temperature at 45-50C. Again, this is on an i9.
So again guys, I have no idea what you are talking about when you are claiming that copying files or doing other basic stuff on this laptop makes it run hot. And yes, I could probably write an app that uses 100% of CPU time to copy some files. Anybody interested? If enough people are willing to pay for it, we might do business.