A civet (assuming hypothetically that's the intermediate host for SARS-CoV-2) infected before Dec 2019 would provide strong genomic evidence of forward zoonosis. A civet infected afterward could still provide that evidence though, if it's by a zoonotic lineage distinguishable from the one that spilled over into humans (if such a lineage exists; you're of course correct it's not guaranteed to).
For now that's all academic though, since we've found zero animals infected by any lineage of SARS-CoV-2, excluding animals that we're quite confident were infected directly or indirectly by humans, from the timing, circumstances, and genomic evidence. I don't think your scenario is impossible, and the traders themselves also had good incentive to hide or dispose of suspect animals before anyone could swab them. There's inherently no direct evidence for it, though.
It seems like most arguments over the origin of SARS-CoV-2 eventually reduce to arguments over what the Chinese government has concealed, whether deliberately or accidentally as part of their containment protocol. That makes the argument hard to resolve, though there's still considerable unexplored evidence within reach of American or European subpoena, like a search through all available raw sequencer reads between September and December 2019 for contamination from SARS-CoV-2 or related viruses.