Pointers returned by xlib don't match any existing window IDs

I'm using some X11 bindings to query some window information and later pass it to FFmpeg. FFmpeg expects a "window ID" given in hexadecimal notation.

This notation seems somewhat standard, as it is returned by programs like xwininfo or wmctrl. I haven't found much information about it, but it seems to just be the hexadecimal representation of the window pointer? If I take the ID string given by these programs and give it to FFmpeg, it is able to capture the window correctly:

$ xwininfo

xwininfo: Please select the window about which you
          would like information by clicking the
          mouse in that window.

xwininfo: Window id: 0x2800014 "Desktop — Plasma"

$ ffmpeg -f x11grab -window_id 0x2800014 -i :0+0,0 -f apng -vframes 1 out.png
# works fine

However, if I try listing all the windows in code:

var root = Window.None;
var parent = Window.None;
Xlib.XQueryTree(_display, Xlib.XDefaultRootWindow(_display), ref root, ref parent, out var children);

var ids = children
    .Select(ptr => $"0x{(ulong) ptr:x}")
    .ToArray();

I don't see 0x2800014 in the results (even disregardingleading zeroes), and if I try running FFmpeg on one of those results, it fails terribly:

$ ffmpeg -f x11grab -window_id 0x4400003 -i :0+0,0 -f apng -vframes 1

# snipped for brevity

Trailing option(s) found in the command: may be ignored.
[x11grab @ 0x55b811a8da40] Cannot get the image data event_error: response_type:0 error_code:8 sequence:10 resource_id:167772160 minor_code:4 major_code:130.
[x11grab @ 0x55b811a8da40] Continuing without shared memory.
[x11grab @ 0x55b811a8da40] Cannot get the image data event_error: response_type:0 error_code:8 sequence:11 resource_id:71303171 minor_code:0 major_code:73.
Input #0, x11grab, from ':0+0,0':
  Duration: N/A, bitrate: 38361 kb/s
  Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 200x200, 38361 kb/s, 29.97 fps, 1000k tbr, 1000k tbn
At least one output file must be specified

So I must conclude my guess that they are hex pointers is incorrect, or that the Window type is not the pointer itself, but then the question stands, how can I get the actual window IDs so I can pass them to FFmpeg?