The depressions look as worn as the surrounding rock and are even in size.
The rock at Joshua Tree is large-grained and crumbly. I believe it erodes quickly, and that people could easily make these depressions (now and thousands of years ago), but I saw no scratches or evidence hinting at human hands at work.
The depressions are in a popular camping and climbing area called Indian Cove. Joshua Tree has other Native American artifacts, so long-term human occupation is not in question. In the following photo, I noticed other patterns. The main group is in the center, but lower to the right you can see an alternating pattern of depressions:
Here is a zoomed in view showing a right-left stepping pattern:
I see many natural depressions in the rock at Joshua Tree, but I've not seen any that are so consistent in shape as these.