whatnot - A: (coll.: indefinite thing) Dingsbums, das (ugs.) B: (stand with shelves) Etagere, die
whatsit - (coll.) (thing) Dingsbums, das (ugs.); (person) Dingsda, der (ugs.)
It may be the definite article 'DAS Dingsbums' that's misleading, since LEO and Oxford-Duden both use a format in which the article serves as the case marker in the dictionary listing. Oxford-Duden's note '[INDEFINITE thing]' (in contrast to 'whatsit [thing]') seems to be a muddled attempt to show that the article is not actually used in context. (@Paul: Next time could you please quote the entry you refer to? Thank you.)
I agree with Harald, minjong, Peter, and gp that 'whatnot' does not refer to a single known object (except in the sense of the piece of furniture, a translation that should be added to LEO). A helpful contrast to illustrate and confirm the point is a web search for 'and whatnot,' the normal and widespread usage, vs. 'a whatnot' or 'the whatnot', which scarcely appear except as proper names or apparent mistakes. Another way to understand it might be just to consider 'whatnot' as an indefinite pronoun, not a noun at all, similar to 'whatever,' 'anything,' 'the rest,' 'all that,' etc.
Given LEO's format constraints, it does seem that 'AND whatnot = was es sonst noch gibt' under Wendungen und Ausdrücke is clearer than 'whatnot = das Dingsbums.' So it might indeed be more helpful just to delete the latter entry, and perhaps to add another option or two for the phrase 'and whatnot' instead.