The market resides in the heart of the city so it could be that there was some other route of transmission through carriers without symptoms, animals etc. To claim that this came from a lab certain requires more evidence than its mere existence. The whole RNA sequence is available online and so far no expert to my knowledge has claimed it looks synthetic.
This is no longer true. See:
'Uncanny similarity of unique inserts in the 2019-nCoV spike protein to HIV-1 gp120 and Gag'
https://www.biorxiv.org/content/10.1101/2020.01.30.927871v1
These are findings from IIT researchers who have found "uncanny" similarities between nCov and HIV which could increase virality.