Even just it's proportions alone show it's not close to being anything like an IS Coupe, it's much bigger. A Lexus IS Coupe would have to be smaller, a compact performance coupe, not a as big as this:

Furuyama can say what he wants, but the RC's platform architecture is straight from the IS. The RC is an IS coupe.Speaking of the IS, in asking Furuyama why Lexus wouldn’t use the brand awareness of their most popular sedan and just call this the IS-Coupe, he explained, “One thing I want to emphasize, the aim of introducing this new model RC is to add a new level of excitement to the existing Lexus lineup. This is not an IS Coupe, it is the RC, a new coupe in its own right. It is an independent model. It’s not a deviated version of the IS so it has its own unique styling and its own unique performance.”