If there had been an ancient technical civilisation on Earth, there would be evidence. Given how flimsy some critters were that did get fossilised, for example dinosaur eggs, some types of sea shell, insects even, preservation of some remains would be likely. Metals would oxidise, but would leave unusually concentrated zones of elevated concentrations, and in un-natural patterns. for example, deep sea shipwrecks will oxidise, but there will always be a localised abundance of metals in the approximate shape and size of that vessel, unless tectonic process distort those sediments. However, certain inert metals, such as gold and platimum would not degrade, however may be crushed and distorted.
also, any technically advanced civilisation would likely have left evidence of mining in the form of a network of backfilled tunnels. since even ancient trace fossils of burrowing animals have been preserved in locations, again one would expect evidence.
lastly, one would expect to see a depletion in fossil fuels older than a certain age, as a previous technical species would likely have used a lot of this material up , with only newer fossil fuels forming in the millions of years since.
End real life section
if the Voth were truly from Earth, it would therefore seem more likely that they were transplanted by a visiting species, perhaps whilst at a stone-age level of advancement, and continued their development in the DQ. Think of the relocation of that group of Boraalans by Nicolai Rozhenko. If they ever developed into an advanced civilisation, eventually, their scientists would notice a paleontological gap, with no direct ancestors, unlike humans, where there is a direct evolutionary lineage.