I don’t think the current virus would need to look different. For the other first few cases in France a month later, we are relatively sure that they come from China because their RNA has mutations that have nit been there in December. If there would have been transmission in France in December, we would expect to see a strain if the virus without the mutations that happened in China in January (but potentially with different mutations).

This makes me think that either

1. nobody else in France was infected by the spread that they now found to have happened in December, the strain died

2. the findings are wrong

1 seems pretty unlikely. We’ll know once they hopefully sequence and publish the genome of the virus they found.