I suppose that could happen if what they were carrying wasn't capable of human transmission yet, but was capable of infecting some species of the live animals at the market, but that's a big if.
There's till the problem of the second lineage which would indicate multiple zoonotic crossover events.
Or if infected carrions from the labs were sold at the market – I suppose it only takes a low-ranking employee wanting to make a few bucks.
All of the evidence people argue over can fit together.
Original virus was brought from the south of china, was studied in the lab. Unclear if it was engineered. Somehow in the lab an animal was exposed. Either purposefully for study or accidentally. Animal dies or will die soon so a rouge employee takes it to the market to sell the meat for some pocket money. First cases show up in lab employees they are smart enough to quarantine. Full outbreak starts at market from animal source.