This was common back then. The PDP-8 had eight primary instructions, although one of them was multi-purpose. This helped keep the hardware simple when we didn't have ICs.

RISC isn't about having a small ISA but about a regular, orthogonal instruction set.

The FDS CPU is very RISCy in nature, but it actually has close to 40 instructions! https://forum.nasaspaceflight.com/index.php?topic=9476.msg26...

And it has been compared to the PDP-8 like rahen suggests, although the Voyager computers are built from medium-scale ICs!