The website you linked to do is an "evangelical ministry is dedicated to Roman Catholics who desire to know how to be saved. It is our sincere desire to proclaim to you the Good News of Jesus Christ, because the Gospel is the power of God to salvation for everyone who believes." That's straight from the top of their front page. They even include the emphasis on evangelical. So it's not Catholic in origin. Their self-proclaimed mission is to "evangelize" Catholics.
It is true that the dogma was not defined until 1950, but it was a belief already present in Catholicism. Things only get proclaimed dogma when essential beliefs start to be questioned. When the Church declares it dogma, that ends all debate on the matter. The Orthodox have a similar belief known as the Dormition of Mary, so this goes back even before the Great Schism.
Suffice it to say, the Catholic Church has a different idea than justforcatholics.org about the history of the dogma of the Assumption. To say much more than that is floating into the realm of debate. If you want to have a full debate on the matter, you can probably get plenty of replies in the theology forums from both sides of the issue.