Rare as. Black Wallabies (Swamp Wallabies) have been recorded eating animal tissue but EGKs are not known to do it. However they avidly dig for salty soil in many locations (generally unrecognised). And when food is short they eventually start attempting to eat a wider range of things, most of which would probably use more energy to process than they provide (same as some starving humans). My guess is that the kangaroo found the bone had a slightly salty flavour. I wouldn't assume this behaviour reflects only the dire food shortage they are experiencing. That is, it might happen in good conditions too, so strong is their desire for salty things.
Yes i thought there must have been some salty taste to it. When i go fishing up in the mountains after fishing down the coast they lick our rods and boat to get the salt off them, even had one jump into the boat to lick the inside. One time camping in the mountains we returned to our camp to find the Skippys eating our eggs :0