I heard that the LCMS believes other Christians aren’t worthy to commune with them. Is that true?

No.  A person does not need to become worthy to take the Lord’s Supper.  In fact, it is the Lord’s Supper that makes a person worthy to stand in the free gift of righteousness before the judgment seat of Christ.
At St. John’s we believe that in the Lord’s Supper we receive the very body and blood of our risen Lord, Jesus Christ, eaten and drunk with the bread and wine, for the forgiveness of sins unto eternal life. Because this great promise from our Lord is given to us together, as a community, at St. John’s we confess that receiving communion at a congregation puts a person in fellowship with that congregation, as Paul teaches in 1 Corinthians 10:16-21.

For this reason, visitors are encouraged to speak with a pastor before participating in the Lord’s Supper that we may teach the fullness of what we believe.

Members of the Lutheran Church-Missouri Synod who have already given their public testimony that they agree with our teachings by virtue of their membership in another LCMS congregation are welcome to commune with us, not because they are more worthy, but because they are already members of our commun(ion)ity.