[CarbonMike]

"......the HF bands were genuinely much quieter back then? "

There is anecdotal evidence that the HF bands were quieter prewar- the old timers in our radio club used to reminisce about it. One of them had been a Marconi Marine operator and reckoned he could regularly work UK coast stations from Australia with just a few watts. However I think your point about thermal noise is still valid.