There are at least 2 reasons why this should not be considered an ex cathedra definition that would fit the definition of papal infallibility from Vatican I.
1 Somebody has already mentioned that the language is not appropriate. This is not the language used in the Apostolic Constitution Munificentisimus Deus : by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma. While it is binding, it is rather a limp statement of the fact.
2 More importantly, it is not really about faith and morals if we read it as a statement about the physical generation of humans. This is why the concept of true human is developed, to make it a theological statement and not historical. It is Pius XII who merges the physical and theological issues, and when he does that he is setting up some of the issues we have discussed. His joining of the two makes sorting the component of faith from the factual statements complex and important.