Yes , your good to go. The difference is, mortar is laid 1/4 "+ thick, sets up solid. It must be broken apart.
Clay is used as a slip, rarely more than 1/8" thick and is easily knocked apart , rehydrated and reused.
Refractory mortar can be used as a watery slip, dipping the bricks and setting them immediately. It is as Thomas says permanent; it is also the required bonding agent for masonry heaters built to code. Fireclay slip is not acceptable by code, if that matters to you.
For structures on the scale of a typical RMH, fireclay would work fine. If you plan to build a bell-style heater with a tall cavity, I think it would be distinctly safer to use refractory cement. Fireclay is strictly a bedding and gap-sealing agent, lending no structural strength beyond dry-stacking.
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