Yes, your semantic view is almost correct.
The UML 2.5 standard explains us in section 15.1 that activities are made of activities and actions:
An Activity is a kind of Behavior that is specified as a graph of
nodes interconnected by edges. A subset of the nodes are executable
nodes that embody lower-level steps in the overall Activity.
The only kind of executable nodes in UML are Actions (...). Actions
are required for any significant capabilities of Activities.
Section 16.1 further explains that:
An Action is the fundamental unit of behavior specification in
UML. An Action may take a set of inputs and produce a set of outputs,
though either or both of these sets may be empty. Some Actions may
modify the state of the system in which the Action executes.
So yes, an action is elementary. But it is not necessarily atomic, as explained in section 220.127.116.11:
However, an Action execution may also result in the invocation of
another Behavior. An
Action is therefore simple from the point of view of the Behavior
containing it but may be complex in its effect and not atomic.