VNC-remoting an X desktop isn't X11 forwarding an application; the mechanism that is moving pixels to the screen where you're looking at them is distinct, and different.
I understand that: I remember when X11 was The Thing (I used to admin Sun sun4m workstations) and I remember when VNC was first invented by some folks at AT&T.
But this is an "XY problem" situation: what you want to get a GUI app running on a remote system to have its window display on the screen in front of you. Xvnc can accomplish that.
But this is an "XY problem" situation: what you want to get a GUI app running on a remote system to have its window display on the screen in front of you. Xvnc can accomplish that.