Question about rasterization documentation

[SleepTight-Cat]

Hello.
I'm a bit confused when reading the documentation.
In the documentation of rasterization, I saw the below formula:

However, in the paper referenced by 3DGS, I found the below:

In this paper, $t_k$ is the coordinate of a point in camera space. This seems to indicate that in the documentation it shouldn't be $t_x$ and $t_y$, but $x$ and $y$, as $(x,y,z)$ is the camera coordinates.
As I'm just beginning to learn this, I may have made some mistakes. Please feel free to point out anything that's wrong.