What? This makes no sense. He was quite familiar with the common understanding of heat, and of course expert in its laboratory applications. He gives careful explanations of the common experiences of heat in terms of the passage of caloric between bodies.
I don't know why you would want to attribute the idea of caloric to some experience or anticipation of a plasma state. The closest thing in our modern understanding to caloric ( i.e. the concept of heat as a fluid substance ) is enthalpy, the thermodynamic state function with symbol "H", which we always jokingly said stood for "Heat".
The point is that "caloric" was in our terms a misconception of which, for very understandable reasons, Lavoisier was never disabused, in spite of his belief that he was dealing in Facts of Nature.
This was a wild guess on my part. What do you think of his other assumption, of oxygen defining acid?