Not unplausible at all. Has happened before https://en.wikipedia.org/wiki/List_of_laboratory_biosecurity...
And this is precisely why BSL4 labs exist. And accidents also happen at BSL4 labs. But in the west, at least we have reasonably transparent and trustworthy governments, with some notion of accountability for enforcing rules.
After working in a BSL3 lab, I've seen some fairly poor working practices including someone infecting themselves, presumably due to sloppy working practices. There is a reason why lab leaks are common, if not vastly less consequential than the lab leak under discussion. It's because people are people, and they make mistakes, whether that's accidental or deliberately breaking the rules. It does make one question whether we can work on dangerous pathogens safely. I'd have to say, after direct experience of work on multiple diseases of varying types, that for many of them I have serious doubts. When it comes to Gain of Function, I think it should be banned worldwide. We aren't capable of working to the required level of stringency to guarantee safety.
To provide a concrete example, look at the German researcher who infected herself with Ebola. In a BSL4 facility. Not even involving GoF research, just the regular virus. Even top researchers slip up. This one made the news due to the severity, but how many are quietly buried, or not even reported within the organisation at all for fear of the consequences. It happens, and I've seen it first-hand.