You do realize that the Article is speaking in terms of "the Church", right? This is the invisible Church, not any one particular church here on earth. This Church we do all have unity with, in heaven.
But, since we don't believe the same thing about communion as, say, the baptists, we don't commune with them. Communion is also a statement of fellowship, or agreement, with our church's teachings. If I don't believe what the LCMS teaches, why would I want to partake of something that makes it appear as though I do, and vice versa.
Be honest here...what exactly are you trying to figure out? WELS and LCMS have long had their differences, but we've always had the attitude that DaRev posted about above, that we would love to have fellowship between the two synods. That just isn't possible at the moment. I'm not sure why you're having such a difficult time with it.