Does opto-isolation matter when there's only one power supply?
Your concerns are not ungrounded**.  Lack of ohmic isolation of power supply and ground rails defeat the purpose of the opto-isolator.  Ground loops remain unbroken.  EMI can propagate through power and ground rails.

Often, there is an isolated DC-DC converter, which provides ohmic isolation of the power supply and ground.  Here's just one example of such power supply, from the dozens of models that are available.  You can also roll your own.

** pun intended