Rather than a "real" instruction that CPU designers consciously created and which was meant to do something useful but wasn't documented, it could just be that this is an illegal instruction and the logic in the CPU is doing whatever it happens to do when given don't-care inputs. (Maybe this is what the author meant, and I'm just catching up.)
Normally the CPU would detect illegal instructions and cause an exception. This would mean there are certain situations where it doesn't.
I found a manual at https://www.nxp.com/docs/en/reference-manual/MC68030UM.pdf. On page 8-9 of the manual (which is page 276 in the PDF file), it says:
> An illegal instruction is an instruction that contains any bit pattern in its first word that does not correspond to the bit pattern of the first word of a valid MC68030 instruction or is a MOVEC instruction with an undefined register specification field in the first extension word.
Note "in its first word". According to the write-up, the instruction is 3 words long. The first word is normal, and the weird bits occur in the second word. So quite possibly the 68030 doesn't validate this second word, just plows forward with the logic that implements the CAS instruction, and lets whatever happens happen.
(Great write-up and amazing dedication, by the way!)