Apparently it is more difficult to train greyhounds to sit than other dog breeds (Google greyhound + sitting and you'll see plenty of results). Something about their body structure, it's not natural, blah blah blah. When we got Molly at nearly-4 years old and were told greyhounds could be trained to sit but that it was difficult, I think my husband took that as a challenge.
With some physical assistance at first and plenty of treats, she was sitting in no time. And ever since she learned to sit, she sits all the time. It just looks a little goofy because her butt hovers an inch off the floor.
Anyway, I have not heard of this comparison before, but after looking up kangaroo pictures, I can see the resemblance. You decide: