It can absolutely be a GPU or cable issue. If the actual device outputting the signal, be it a motherboard when using onboard graphics, or a dedicated GPU only has a HDMI 1,4 output port, you will be limited to 4K30. This is a hardware limit and not something you can just apply in the GPU control panel. As you can see in the below spec page from hdmi.org HDMI 1.4 can only support 3840x2160 at 24, 25 and 30hz. So if the user has a older GPU, which can be quite common in the emulation community as current high end GPU's are not required for most emulation then they simply may be hardware restricted.
https://www.hdmi.org/spec/hdmi1_4b