It would certainly explain how the predecessor candidates simultaneously acquired all of the necessary mutations: all of the most genetically similar coronaviruses were all being studied in the lab; if precautions were not taken to avoid cross-contamination then they could picked up the mutations in a relatively benign environment without external stressors. (One of the initial reasons given for the man-made theory was due to the fact that COVID19 was unlikely to arise naturally in the wild because any combination of mutations among the predecessor strains short of the entire package of mutations that ended up in COVID19 would have resulted in a strain less able to reproduce than existing competing strains.)
Similarly, lax attention to safety practices would also explain why the first known cluster of COVID19 cases occurred among Wuhan lab workers.
But the only serious contender for the prior hypothesis is that the researchers produced a highly infectious chimeric coronavirus through gain-of-function/serial passage.