I did not see official support for << Stereotypes >> in legacy or beta Activity Diagram support, but I got an unexpected error when using "<< Foobar >>" as the activity/action name.
Test cases, one of which resutls in unexpected error.
Test Case 1:
@startuml
start
: << Automobile >>;
stop
@enduml
Behavior: No Error
Test Case 2:
@startuml
start
: << Automobile >> Pinto;
stop
@enduml
Behavior: No Error
Test Case 3:
@startuml
start
: << Automobile >>
Pinto;
stop
@enduml
Behavior: Syntax Error
Test Case 4:
@startuml
start
: << Automobile >>\nPinto;
stop
@enduml
Behavior: No Error