They do have an R8 core, but they don't have a trademark on that part of the name[1], they just have one for Cortex. However, the name should probably be changed for removing confusion anyways, because Renesas has an R8C series of CISC microcontrollers [2]. The namespace around single letter and then a number processor model is real crowded.

[1] https://www.arm.com/about/trademarks/arm-trademark-list/

[2] https://www.renesas.com/en-us/products/microcontrollers-micr...