seems funny to me, but it looks like you just answered your own question, Phil. Worm bin aficionados tend to observe that onions and citrus do not get consumed by their worms, which are 'composting' worms, and since this is true, they assume that these items do not compost. They do compost, but not via worms. A half lime squeezed of liquids and added to a pile, might be in the pile for a while, or in several piles, before it composts fully, but it will compost as will onions. If it didn't, my composts would be a shambles (which is not the case); at the moment the compost heap is about half onion tops, skins, and rejects from a massive harvest that is less than half way done. I toss some citrus peels in with my urine bucket to aid with odors; this also seems to help the peels break down a lot faster. But I wouldn't worry too much about oranges and onions. Just make your heap, layered with enough carbon and nitrogen, and turn it when it cools a few times, and you will break most of anything down (as long as it's not too bulky). I sift my compost, and get things like broccoli stalks, avocado pits, citrus peels, and such hard woody materials on the screen, and these are either used as mulch or added to a future compost process.
I have no idea where this information germinated, but it's erroneous with respect to compost (worms do seem to hate citrus, though).