You don't see anything odd about the facts that Barcelona want to sign him, and you thinking he is easily replaced?
Well, as SuperJackWilshere19 said, the sources aren't very strong, and I also highly doubt that Barcelona actually want Song, and if they do, he will just be on the bench there. Barcelona need depth, just like every other team, Song's quality is enough for the level of Barcelona for him to be a bench player.
It's not the first time Barca have shown an interest. I'm pretty sure those rumours are accurate. Songs a TOP player , massively underated.
I doubt Song will leave this year, because he has two years left on his contract I think. What it may be is a statement to stop him signing a new contract with Arsenal, putting him on alert for next year. I wouldn't put that past them.