The Morris book, while not a bad popular reference in its day, is quite old. A lot has been discovered since it was written.
What constitutes slang can be rather slippery, and there are various definitions of what slang is. My favorite is from J. E. Lighter’s Historical Dictionary of American Slang:
an informal, nonstandard, nontechnical vocabulary composed chiefly of novel-sounding synonyms for standard words and phrases (1:xi)
Lighter includes kangaroo court in HDAS, so presumably he believed it fit the definition. It is informal and nontechnical; it is novel-sounding; and the standard term it replaces is irregularly constituted court, or a construction of that nature.
I’ve edited the Big List entry on kangaroo court. I think Barry Popik has found the origin, but we need some evidence to firm it up.