We can obviously assume he copied it because of the repeated hints coming from the author, or editor notes along with Naruto's (Kishimoto's) implications.Because Kekkei Genkai were explicitly stated to be uncopyable. Besides, who said he copied it? Naruto. How the **** does HE know what happened?
Kekkei genkai cannot be copied because it requires a person to have the bloodline inheritance or what have you. If the requirments were met, why would someone not be able to copy and perform?
This has been disproven before. Madara used Hashirama's techniques after recieving his DNA. Why would it not be the same for Kakashi who had recieved Obito's eyes and chakra?