Yep. That's the case because freight cars aren't equipped with marker light brackets. The FRED also monitors the air brake pipe pressure.
But that still doesn't mean all cabooses had flashing red lights.
I'm sure not all did, and I don't know if they flashed or were solid, but there are plenty of photos on line showing single red rear marker lights mounted on the edge of the roof and in some cases right below the running board on the cabooses that had them, just like the models. During a brief look on line I found examples from WM/Chessie, Santa Fe, Milwaukee Road, MKT and Great Northern, among others.
There were a number of comments from TrainOrders about single red lights on the rear of cabooses, like this one:
"The side marker lights were usually either red and green or red and yellow. They were in use until about the mid 1970s when just a single red light at the rear of the caboose came into acceptance."