Maybe I've misled you.

You are definitely correct in your analysis. If you perform a citric flush, the acid will have a beneficial effect on the internal passages of the heater core.

However, my comments are that the heater core can function well even when it is not in perfect condition. If the remainder of the system is marginal then it makes sense to perform the flush, which will also serve to flush the heater core.

But, if the system is working well, it does not make sense to go through the effort of a citric acid flush and/or replace the heater core.

Would you make the effort to replace the evaporator just because it is old? Now here is a component that could use a good cleaning. But the cost and time is very high.