I think one person opened the machine, saw the 8MHz XTAL and assumed the CPU is 8MHz. And this machine is so rare, that nobody verified the clockspeed.

It's the same thing that happened with the manufacturer information. Someone in the past assumed it's by Applied Concepts/Chafitz(why?) and every other website copied that information. The company that made this machine is France Double R. ACI/Chafitz were American.

Anyway, CPU is a Z80A.
Z80A = max 4MHz
Z80B = max 6MHz
Z80H = max 8MHz