What the idiot is actually talking about is the AC's heat exchanger, which is in line with the furnace in a central air HVAC installation and usually directly above it and 'Downstream' in terms of airflow. It will accumulate dust and fiber over the years, particularly if one runs the system for an extended period with a broken filter or no filter. Avoiding having to disassemble the ductwork around the heat exchanger (Everywhere I've lived, it was accessible by removing screws and tape without cutting metal) is the primary reason to check and change your filters every so often.
The normal way the grunge causes HVAC failure is by holding water condensed from the air against the cooling fins, where it freezes into a solid sheet of ice in maximum AC season, preventing any air at all from going through. If it was actually nasty enough to prevent HOT air from going through, it was three times worse than any I've ever seen and it would have also been a fire hazard.