Your hope is the actuality. If you have $LTIGradeMode = "";, then no mass update will occur, and you won't see those messages. If you are still seeing those messages for users in a particular course, then that course must have $LTIGradeMode = 'homework' or $LTIGradeMode = 'course'. Check the courses course.conf and simple.conf files.
Looking closely at the code I see that if you have $LTIGradeMode = " "; then mass grade passback will still occur, and you might still get those warnings. You definitely do NOT want the space.