Notice in your link that in various tests the slower G4 was faster than a higher-clocked PIII. The P4 is slower per-clock than a PIII. Complete the logic.
In your Digital Video test, the G4 was hampered by a new, unoptimized operating system. In those days the OS's GUI sucked up a lot of processing power, before it was later accelerated using Quartz Extreme. Speed improvements since then have OS X being much more efficient -- they make it faster every release (unlike another OS I know).
But none of this really matters for anything, and can only be guessing. I'd need this Apple machine, a Celeron and a Sempron all together to find out how the Apple stacks up against the competition.
And it really doesn't matter because in this market speed doesn't mean much. Small, cheap, quiet, low-power consumption and able to do basic jobs is what I'd look for.
I'd characterize that page as more of a mixed bag - here are results, IIRC, on that page showing a PIII-500 comfortably exceeeding a G4 at the same speed. Anyway, I don't know how all this got going, but I do agree that this machine is not intended for the speed obsessed. If it turns out to exist at all, that is ;)