There are multiple examples of novels that are little over 40,000 words. Ray Bradbury's Fahrenheit 451 is 46,118 words. Douglas Adams's The Hitchhiker's Guide to the Galaxy - 46,333 words. Erich Maria Remarque's All Quiet on the Western Front is ~60,000 words. All are, rather obviously, novels. (Source) Which indicates that your assumption that a novel needs to be over 80,000 words is mistaken.
For shorter works, the source you provide is a good guideline. In particular, this is the guideline followed by the Hugo and Nebula awards, (but notably, other awards use slightly different upper margins on a novella's length. Source) That said, Hemingway's The Old Man and the Sea is routinely featured on "best novel" lists, although it is merely 26,601 words - on the lower side of the 'novella' length.
I have seen the term short novel used sometimes, to describe a work that's comparatively short for a novel. However, I found no formal definition of the term.