They are not saying the assembly is bad all the time, but merely that sometimes, for some inputs, the assembly is bad. Which is definitely true, though I would prefer the term "suboptimal."

Again, to the OP's point, look at any of the ultra optimized video codecs. You will often find hand rolled assembly to squeeze out that extra bit of perf; having the compiler do its job would probably emit something that does the job, but you can absolutely spend the extra time to do it yourself and do it better.