So same thing for etymology. If the etymology of a word is the tortured trail it has taken to get from an ancient clump of phonemes to the clump of phonemes we use today then no, the phrase untraceable etymology is not an oxymoron, but if it is the map of that trail as it is laid out from written evidence or linguistic SWAGs by trained professionals then yes, it is an oxymoron.

NB: oxymoron is itself an oxymoron.