Boanerges,

1. What you have quoted from is not Luther's Small Catechism. It is an explanation of Luther's Small Catechism prepared by the Lutheran Church-Missouri Synod. I, as a Lutheran, am bound only to the symbols of my church as contained in the Book of Concord, 1580.
2. The sentence you have quoted makes no sense. If baptism is the only means, how can there be any exceptions?
3. Luther's Small Catechism does teach baptismal regeneration (page 17, part III) but it does not teach that baptism is the only means whereby infants can be regenerated and brought to faith. That teaching contradicts the Lutheran Confessions in many places.
4. Contrary to the Missouri Synod explanation, the Confessions speak of Word and Sacraments as the ordinary means of grace not just baptism. Is infant baptism the ordinary means for Baptists who wickedly withhold baptism from infants yet permit their children to hear the gospel?
5. Not only does the Missouri Synod explanation make no sense and contradict the Lutheran Confessions, it contradicts itself:

Quote
Q. 256. Can anyone be saved without Baptism?

It is unbelief only that damns; and though saving faith cannot exist in the heart of one who refuses to be baptized, it can exist when for some reason Baptism cannot be obtained.

The 1943 Missouri Synod explanation is a good example of how doctrinal error is propagated in a confessional church. Fortunately, the error was caught and corrected in the 1991 revision. The offending sentence has been deleted and the explanation of infant baptism is now in agreement with the Lutheran Confessions.