The gist of the answer is hiding in plain sight: it took so long, on an exponential cost function, that they couldn't afford to explore any further.
The better their max demonstrated accuracy, the more impressive this report is. So why stop where they did? Why omit actual clock times or some cost proxy for it from the report? Obviously, it's because continuing was impractical and because those times/costs were already so large that they'd unfavorably affect how people respond to this report