That is the way I look at it. The HTF is more than just a secondary source of feedstock.  It is a value added feedstock as it increases the throughput and the anti caking may be a benefit as well.

It would make sense to me that the HTF is probably pre-heated first and then injected into the pre-melt. This would dramatically increase the thermal transference inside the premelt.

To go back to the old Ice cube analogy, if you put Ice cubes in a pot on the stove the cubes will melt pretty quickly, but if you add some hot water at the same time the cubes will melt much much faster.  I can see the HTF as having a major impact on throughput.