Dranov was not a mandated reporter in that instance.
It's all really simple. MM saw Sandusky and a kid in a shower. That freaked him out (as it should have). He assumed bad things were going on simply due to context (fair assumption) even though he himself didn't witness any bad things.
When you look at the actions of everyone involved, that's the only explanation of how it went down that night. You take away the "you know what" inclusion in the presentment, then all this goes a different direction.
Far from the "only explanation."
Equally plausible: Regardless of what Dad & Dr.D may have believed actually happened, by the time they were told (whatever) they knew there was no immediate threat & whatever happened was over. They decided to report to Penn State, through Joe the next day and let them carry the ball forward.
In no way does that indicate what Mike observed, what he told them, or what they might have believed. They just advised him that PSU could take it from there, tomorrow. Joe also seemed to agree that was an ok approach.