There are no joints to be seen within the arms assembley, only fabric.
But at the lower end of the arm there is the wrist ring.
This ring is used for a gas tight connection of the gloves to the suit. But there must be another function of this wrist ring, it should allow the rotation of the hands.
I remember a video of the suiting procedure of an Apollo astronaut, he tested carefully the rotation of his hands, the movement was smooth and easy.
But if this wrist ring and its counter part in the gloves are contaminated with moon dust, rotation of the hands may be impaired. Between these two metal rings there must be a O-ring seal and some of the lubrication of these O-rings might been lost due to the vacuum during EVA. The remaining lubrication may have mixed with moon dust making rotation difficult.
All images from a NASA page about Buzz Aldrin's Apollo 11 Suit
In this report about space suits evolution rotary bearings are mentioned in the section about Space Shuttle suits on page 13:
The gloves incorporate a rotary bearing to allow wrist rotation,
In the section about the Apollo suits on page 7:
An example of this stiffness: inflate a large cylindrical
balloon or the inner tube of a tire, the balloon or tube
will become very stilt and almost impossible to twist or bend.
Without these specially developed joints for the space suit, It
would be virtually impossible for the astronaut to do useful
work on the moon's surface. These special joints are installed
Into the CMP A7LB suit at the knees, wrist, shoulders,
elbows, ankles, and thighs.
From this PDF APOLLO OPERATIONS HANDBOOK EXTRAVEHICULAR MOBILITY UNIT on page 2-23:
The glove side-wrist disconnect is the male portion of the wrist
disconnect assembly and features a sealed bearing which permits 360°
Figure 2-15 from page 2-44.
Lubricants were used for O-rings (page 2-78):
The lubricant pouch assembly contains eight fluorinated, oil
saturated pads which are used to lubricate pressure sealing slide
fasteners, seals, and "O" rings. The pads are held in place in the
center pouch of the pocket assembly by whipstitched Beta thread.
So a rotary joint is located in the wrist disconnect at the glove side.