What was gained that would not have been gained without the existence of the lab?

You really missed the whole point of their post didn't you? If we are going to allow a BSL-4 lab to exist and introduce the risk of possible leaks or other problems, we need to get something in return. That "something" needs to be more than what other lower security labs can already do. It is fascinating that a lab which was supposedly researching coronavirsuses for so long (and which may have led to the outbreak, but regardless) was unable to do anything that other labs could not.