In my view there's a few possibilities
1. - You left some cambium intact and the tree has reconnected the bottom/top half with live tissue. Seems unlikely as there is no visible presence of reconnected live tissue in the picture you showed but we also cannot see the backside so can't completely rule this one out. It would be very easy to see if the live tissue has healed enough to bridge the gap between the top/bottom. In that scenario, you have to re-girdle the area and keep going.
2 - You didn't wait long enough. Callus has clearly formed which is good. Whether that callus will ever produce roots is a question of time, species and maintaining a root-friendly environment immediately surrounding the callus. Time only helps if you have a root-friendly environment readily available. So you need to keep the air layer wrapped in well-hydrated sphagnum and just keep it going and you will eventually rule out whether the callus will produce roots given enough time. It could take multiple growing seasons, but as long as the top branch has living foliage there's no reason you can't keep the air layer going.
3 - The species will not form roots from callus tissue. As many others have mentioned this is very likely to be the case. If it is a genetic limitation then no amount of time will change the outcome. However I love the idea of pursuing it anyway. Since the branch is already girdled there's not much hope for that branch long-term anyway. Just keep in mind even if you did get the air layer to successfully root, the tree will be weak and slow-growing on its own roots. It will not grow vigorously like the mother tree which is grafted onto more vigorous root stock.
What's the big rush? Leave it alone till next Spring potting season. Then check again. These things don't normally happen over night.
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