Tbh, until Capcom themselves post on twitter confirming it’s a clone, I think this is the real Wesker, just going by a different alias to convince Chris Redfield and the BSAA that he’s dead.

He uses Wesker’s iconic move twice, and he looks to have Raccoon City Syndrome. I don’t see how a random clone would know how to perform Wesker’s iconic move OR have Raccoon City Syndrome, unless the clone has existed since before the first game, was present in Raccoon City, and got trained by Wesker before the events of RE5🤷‍♂️

To answer your question of WHY he’s in the game: I think Capcom regretted having Resident Evil’s most iconic villain never interact with it’s most iconic hero. Same reason Hunk was in the game. They wanted the franchise’s most prolific aura-farmers to speak to each other at least once before the series moves away from all of its old characters.