This is somebody’s pipe dream. A number of the saints mentioned were actually real people in the early Church and we have historical evidence that people like St. Nicholas actually existed and were revered in their time. Moreover, some of the information on there is just plain wrong. They refer to “St. Martin-in-the-fields” which is actually St. Martin of Tours, who was a real person who was a Roman soldier (the son of a Roman military officer) and later a well-known bishop of Tours in what is now France.
The only thing with a hint of truth on that website is the suggestion that sometimes the Christian church made a pagan festival into one that was refocused on a Christian saint or on Christ. This makes sense because the festivals were often, essentially, agricultural or community events, and making them Christian allowed everybody to continue to have the customary celebration but get rid of the pagan part. This is NOT the same as saying we just took some Pagan saint and “invented” a Christian one, when the Christian person is a real person.
The Church in recent decades has been pretty aggressive about removing “saints” from the official records when there is, in the view of the Church, inadequate historical evidence that the saint existed. The Church is fairly strict about who’s allowed to be venerated as a saint, partly to protect against this sort of accusation.
There are some religions I’m aware of that have taken Catholic saints and invented some type of pagan god or voodoo figure based on the Catholic saint, but in those cases, the Catholic saint, who was a real person, came first and the pagan invention came later on.