That tells me that the original term "hotrod" is of unknown origin. A lot of great theories though.
The term "ratrod" is a modern term losely used to describe a built to look unfinished, in the works, but driven hotrod.
Since we can make old parts look new, and new parts look old, I don't think it has a thing to do with a guy recycling parts to do a low buck build. I think a guy could build the rattiest rod he could using all new parts. And a guy could restore a pile of old parts to make a shiny flawless hotrod.