The PDP-5 also offered expansion up to 32K words, similar to the PDP-8. This was the "Memory Extension Control Type 154" as seen on the second-last page of this PDP-5 brochure:
MEMORY EXTENSION CONTROL
Allows expansion of the PDP-5 memory from 4096
to 32,768 words in increments of 4096 words. Can
be attached to any PDP-5 without requiring changes
to the processor.
So the claim that only the PDP-8 had the "expanded instruction set" for this is wrong. The claim that memory expansion required an expanded instruction set doesn't strike me as utterly unreasonable, since IIRC instruction decoding for additional "IOT" instructions was on the cards that added the peripheral, so you could argue that the particular
IOT (sub-?)instruction didn't exist in the machine until the peripheral was added.
Regarding rotations, as well as the standard four (accumulator and link left/right 1/2 positions) shared by the PDP-5 and PDP-8 in operate group 1, DEC's 1974 PDP-8 Pocket Reference Card includes a section called "combined operate microinstructions" that includes among other things two shifts and two additional rotates:
COMBINED OPERATE MICROINSTRUCTIONS (1.2usec)
CLL RAR 7110 shift positive number one right 1,4
CLL RAL 7104 shift positive number one left 1,4
CLL RTL 7106 clear link, rotate 2 left 1,4
CLL RTR 7112 clear link, rotate 2 right 1,4
It's not clear to me if these rotate through the link or just rotate the accumulator. If the former, they're not different in how much gets rotated, but they are different in that they change the data before rotation.
These don't seem to be documented in various other instruction set summaries, so I'm wondering if these were instructions added by optional hardware, rather than part of the base PDP-8.