Yeah, on second glance I might be confusing some of my sources here. I can’t seem to find what I thought I was remembering.

Regardless, there seems to be a significant distinction between words using a concatenated ur prefix and those using a hyphenated ur-(noun) prefix. In this case, the usage does seem to imply an original language from which all others are derived, and the metaphor seems apt even if it is not historically accurate to say that’s where it came from.