I don't know where you are getting "a decade" from, but assuming we are using the percentage of passing tests as our metric of the percentage of "what coreutils does"--which is dubious, but it's your metric so let's go with that for the moment--we see in the very same plot that just four years ago it only did 25 percent of "what coreutils does", so clearly significantly more has happened in the last four years than did in the previous six, rather the project being worked on equally hard for the entire time.

Also, you seem to imply that it shouldn't have taken them "a decade" to get accomplish "85 percent of what coreutils does", but that raises the question: exactly how long should it have taken exactly? Can you cite evidence that it took significantly less time for coreutils to get to the point where it accomplished "85 percent of what coreutils does" today? If not, then there is no basis of comparison we can use to decide whether a decade is a long time or not to have gotten to this point.