Qt has been open source since 1999 and GPL-licensed since 2000, nearly 17 years. GNOME was started in 1997 and GNOME 1.0 was released in 1999. So,
much later is not really an adequate description.
I think the grandparent is definitely right. A lot of projects were started post-1999, when Qt was already open. But GNOME was the default desktop of many distributions (including Red Hat and later Ubuntu). Moreover, it is harder to bind Qt in order languages, since it is C++. There was a strong dislike for C++ in parts of the FLOSS community.