Well the A500 CPU was 7MHz, but it also Co-CPUs like Denise, Agnus, Copper, etc...
The Amiga's sound coprocessor, Paula, would only do hardware level mixing of four 8-bit channels. Octamed faked 8 distinct channels using real-time CPU level mixing, so the original statement above still stands...
(BTW: The copper was a sub compoment within Agnus, not a seperate chip)