If you zoom in you can clearly see the milk teat. I have drawn a red ring around the squished empty pouch for reference. In the centre, the teat is poking out the pouch opening showing how joey is still able to drink even though far too big to get in the pouch. Joey only ever has drunk from that teat. The other teat, unused this year is quite small and the next joey will latch onto that and only use that one. If she doesn't get mated for some time after this one weans, the new joey could take either teat as this one will shrink again once not in use for a while.
Ian