According to Nerdist - Game Of Thrones Animated Short Explains The Conquests Of Westeros
It is narrated by Viserys (Daenerys’s brother who died in season one
but doesn’t seem to know that), Varys, Euron Greyjoy, Jaime Lannister,
Sansa Stark, and Petyr Bearish.
& though I haven't seen it yet, I presume that this means it's portrayed as the characters doing the narration, rather than the actors who play those characters.
Edit: Having just watched it... the entire cast is listed in the opening minute & indeed the narration is by the characters, as opposed to the actors.
 Of course, neither Viserys nor any of the other characters at this point in time know of the events to come, from Season 1 of the TV show. It is all set in the years leading up to when the show starts.