That's not true, AFAIK. The DRM module (e.g., widevine) has to negotiate keys with the host and will decrypt the stream for decoding. The HDCP keys are static and independent from the streaming service. That decryption can happen with hardware support but it can also happen in software. I think most streaming services fall back from 4k to FHD or lower if their "security requirements" aren't fully met.
Also, if the DRM module supports hardware decoding, the GPU will need to see a decrypted stream. The GPU memory is in principle accessible to the driver. So if you can manipulate the driver, or setup another kernel module, you might be able to grab that content.
Also, if the DRM module supports hardware decoding, the GPU will need to see a decrypted stream. The GPU memory is in principle accessible to the driver. So if you can manipulate the driver, or setup another kernel module, you might be able to grab that content.