John is writing to the church ..thus the "Not only ours"..meaning the people reading the letter ...but for those not reading that letter..and of course Jesus died for the sins of the world..the atonement is sufficient for all but only efficacious for those that come to Christ..(unless you believe that coming to Jesus is not necessary for salvation)
It says the "sins of the whole world" so it must mean the "sins of the whole world." Deal with it. Don't rationalize it. All means all. The world means the the world. Them means them. If you put it all together it means that Jesus died for everyone's sins and not just for the sins of the elect.
Now the challenge is whether Calvinism stands or falls now that it is clear that the Atonement was not a "limited" atonement.
Clearly not everyone's sins are "effectively" forgiven, but that is not the fault of Jesus, is it? No! Jesus died for the sins of the whole world. Not just the Church. Not just the readers of the letter. Don't twist the scripture. Read it. Digest it. Deal with it.