The point as I said is to keep loop() from ending. If you won’t add for ( ; ; ) {} then change i ++ to i +=0.
I’m sorry I don’t write infinite for loops often and I’m not next to a compiler.
Do it this one time and you will prove to yourself that serialEvent() is not an ISR. If you allow loop() to end then you allow the function call to serialEvent().
P.S. the main has that for loop in it for you. See above.