No, not necessarily (although I can't answer for them, of course). In my experience, churches that limit communion to their own members usually do so for other reasons, and they aren't implying that no one else is a Christian.
Some churches, for example, give great significance to the Lord's Supper and make it a central part of their worship. They want to be sure, therefore, that those who participate understand its significance, and feel that a casual visitor might not do this. Other churches take very seriously the Bible's teaching that "A man ought to examine himself before he eats of the bread and drinks of the cup," to be sure they aren't guilty of any unconfessed sin (1 Corinthians 11:28). Again, they may feel a casual visitor might not have done this.
Although churches differ in the way they celebrate the Lord's Supper, they all agree that it reminds us of one important truth: Jesus Christ died on the cross for us. As Jesus said when He ate that final meal with His disciples, "This is my body given for you; do this in remembrance of me" (Luke 22:19).
Whenever you share in the Lord's Supper, always stop to thank God for His great love for you. Because of what Jesus did on the cross for us, we are forgiven of all our sins, and now we can look forward to being with Him in heaven forever.