I'm not experienced in GUI at all, but, if you take a peek at every XGUI file in the ulx addons/ulx/lua/xgui folder, Stickly was nice enough to make a file for each of the main XGUI tabs (commands, groups, maps, settings, maps, ...)
Looking at the top few lines of each of those, it seems reasonably easy to surmise what his library command is to create a tab.
Big hint - He calls it a "panel"
And, as I suggested in that other thread you refererenced , the link I provided to those TTT commands also use the same premise.
No, he's not documented xlib, his own personal library of stuff he noticed he kept using over and over in XGUI.
However, one could study and learn many functions of what his XLIB does for Xgui by looking at the 3 ulx client modules for xgui located in /addons/ulx/lua/ulx/modules/cl, including...ta-da...xlib.lua