Are your key words here “fully” or “valid”?
Obviously, no one catechism from any source, be it the CCC, Catechism of Trent, Baltimore Catechism, or that of St. Pius X, can fully exhaust the mystery and depth of our faith.
I’m sure that the Catechism of St. Pius X (or any other authorized Catechism) is true and valid as far as it goes, but is not necessarily supposed to be the last word.
Otherwise, statements of the magisterium (either ordinary or extraordinary) would not be necessary.