It must be a cultivar or p. thunbergii as no one would waste time grafting an ordinary JBP. Maybe it is a corker? The graft is quite young/new, but looks pretty good to me.
Most cultivars of any species are grafted. So using a special cultivar for bonsai effectively means hiding the union from view. Cork bark cultivars are all about the bark, so one wants a very low graft. Then the substrate level is set at the union - a tree with no nebari is a fact of life. Otherwise, one wants, IMHO, the union just below the first branch of the cultivar where it will be easily obscured from view. We frown on grafts with deciduous trees because we prize their bare winter image - there is no way to obscure the graft union.
JBP can indeed be air layered. It is most readily done with a candle from the previous year, but they will produce adventitious roots from several years old wood. George Muranaka, in Nipomo, CA, propagates JBP by layering last year's candle in one year. I have a much cooler growing season and it took 3+ years for me to successfully air-layer a 'Thunderhead'.
But just getting one of
George's JBP layers might be a good option. I assume he's still selling them on eBay - maybe call him directly, if you want.
@PeaceBD .