Solved: I learned, when the wording “read-only” is used here it may simply mean:
The exposed C++ event has the const modifier on the definition / declaration.
I missed that for one event when the parameter list was long (const on the next line). 
Solved: I learned, when the wording “read-only” is used here it may simply mean:
The exposed C++ event has the const modifier on the definition / declaration.
I missed that for one event when the parameter list was long (const on the next line).