From a safey perspective, is it necessary to seal mortar joints between clay liners in the flue of a fireplace?
A single fire could cause health and safety issues if enough of the exhaust gases find a path into the home or heat finds its way to built-up creosote between the chimney proper and its liner.

I would question their method and service-life performance of "sealing" clay liner mortar cracks and compare that, as well as cost, to re-lining the chimney with a flexible, stainless steel flue liner (for its expansion/contraction properties --which tend to break up &amp; drop adhering creosote-- over the rigid format).  Flex liners tend to be about US $100+/ft.