The specific recording falls under a seperate copyright, this is 100% true in the US. And it's not about YG imitating his voice, it's about him using the actual recording of RA in the start of this clip.

Here is Legal Eagle explaining something similar with Taylor Swift https://youtu.be/M-A_RrOeoWw&t=4m36s

For some reason the timestamp isn't working properly but the important thing starts at 4m36s