I doubt the old limb would have any viable buds to generate roots. if it did the new limited root structure would not support fruit for years at best. this rootstock would be of the scion, and probably a standard (big) size. this means slower fruiting, typically. it's the dwarfing of the rootstock that expedites the fruiting, and in your scenario you would not have that. that would be too easy! I have grafted fruiting wood onto mature crab apples and had them produce in the first year, but according to the pundits on this site, that is not very normal either.

I see your point that I apparently missed.

I forgot that the roots themselves need to develop sufficiently to support the large above ground portion of the plant.

But, just as an observation, as I saw on the videos, if you make a large enough cocoon of growing media around the area, the roots grow into the media pretty aggressively.

Air layering in late spring developing a good root mass over the summer and not putting it into the ground until late fall ( in the northeast) will let the roots really take hold by the following spring to support the new growth at that time.

You now have essentially a 4 year old tree. I may try an experiment this year.

Mike