First of all, it's not true. The Church never issued any kind of decree saying that Christopher never existed. Furthermore, competent hagiographers, including Protestant ones, tell us that there was a Christopher, but we just don't know as much about him as some of the legends that grew up around him would suggest.
Second, it would not matter even if there were no Christopher. Papal infallibility applies only to those canonizations that a pope has done. Christopher was recognized as a saint in the period before popes became involved in the process, meaning his canonization is not subject to papal infallibility.
The confusion over Christopher's status comes from the 1969 reform of the Roman Calendar. This reform had been mandated by Vatican II in Sacrosanctum Concilium, its constitution on the liturgy. Because the Roman Calendar was getting crowded, especially with saints with local rather than universal followings, the Council declared: "Lest the feasts of the saints take precedence over the feasts commemorating the very mysteries of salvation, many of them should be left to be celebrated by a particular Church or nation or religious family; those only should be extended to the universal Church that commemorate saints of truly universal significance" (SC 111).
A revision of the Calendar was undertaken after the Council, and on February 14, 1969, Pope Paul VI issued a motu proprio with the unwieldy title "Approval of the Genera Norms for the Liturgical Year and the New General Roman Calendar" (AGN). In this document, which is found in standard sacramentaries, the Pope explained:
With the passage of centuries, it must be admitted, the faithful have become accustomed to so many special religious devotions that the principal mysteries of the redemption have lost their proper place. This was partly due to the increased number of vigils, holy days, and octaves, partly to the gradual overlapping of various seasons in the liturgical year.
The purpose of the reordering of the liturgical year and of the norms accomplishing its reform, therefore, is that through faith, hope, and love the faithful may share more deeply in the whole mystery of Christ as it unfolds throughout the year. (AGN 1)
To put [the] decrees of the Council into effect, the names of some saints have been deleted from the General Calendar, and permission was granted to restore the memorials and veneration of other saints in those areas with which they have been traditionally associated. The removal of certain lesser-known saints from the Roman Calendar has allowed the addition of the names of martyrs from regions where the Gospel spread later in history. (AGN 2)
In the Calendar that this document serves to implement, Christopher's name is omitted. One can question whether Christopher should have omitted. The devotions to him were broad-based enough that they would seem to make him a saint of "universal significance." Nevertheless, nowhere in this reform is it implied that he did not exist or that he was not a saint.