2. Human sensory system takes information in rate of 1 billion bit/s, and our nervous system heavily filters this data to reduce cost of processing data. Human conscious thought rate is
10 bit/s(100 million slower than sensory system). Unlimited Void targets mind directly, AFAIK. 1 second of information directly to your mind is equivalent of 3.3 years of normal informational input. You would have need Rel perception speed to resist UV if it gave 1 second of information per 1 second. But It gives 6 months of direct sensory input in 0.2 seconds. Opponent would need heavily MTFL+ perception to resist this.
In conclusion, Sukuna not resisting UV is not argument against him having Rel perception (there are other problems with it, but not this particular one)