It was his dead brothers skill.. his brother left beloonings in case he died (he vanished not telling Mc anything). Years later Mc gets the item and because they are twins their souls are linked. Its not just the twin trope its the fact that Mc was the stronger of the two and always protected his brother. The brother left the item MC got as a way of saying "sorry big brother i left and neveer contacted, im sad to say if you get this im dead".
Mc wanting to know what happend to his baby bro found info and when he entered a remaining fragemnt of his bother and their soul compatibilit and prob Mc's longing for his brother "gifted" him his brothers power. As Mc is now in the "game reals" his soul is reonating with his brothers and hence he sees his brothers experiences like a movie. + finds notes his brother left behind.
Oh also the manga is Korean and if i remember righ there is a belife in Korea that Twins have the same soul, its just that the soul split into two. So if one twin dies that soul fragment finds and merges with its other half becoming Whole again.
i am so confoosed rn
iv seem a lot of ppl saying that other guy is his dead brother but i dont think so i mean he got all his memorys and that dragon eye power the dead brother had and it was an indivuial skill and plus why woud his dead brother bring him there or send him that stuff he he really wassnt dead