That would explain the delay in supply. The only way he could insure the rootstock to be genetically identical to his selected cold tolerant type would be with clonal propagation. Even if the parent tree provided poly seeds, only select sprouts would be identical. As for grafting material, a single adult parent tree could provide an infinite supply of buds. The only problem with budding is rooted clonal rootstock would be past the juvenile stage making it very difficult to successfully bud graft.
It still is a modern miracle to breed something like mango into cold tolerance without genetic modification even in a 100 year or more program. Its quite a quantum leap. Things like disease resistance, productivity, fiber, flavor, and color/shape would be more typical in a selective breeding program. Its why we don't see tropicals in the subtropics after all.
Mango is a very widely adapted plant, growing and fruiting fine in tropical, sub tropical, and even dessert regions. I wouldn't be at all surprised if it's growing range could be extended even further than is the case at present, as it is a very versatile plant, not an ultra tropical.
Mango is subtropical Oscar, that's why it doesn't do as well in areas that ultra tropicals thrive. If it were ultra tropical we would be struggling to make it grow in Florida. Its actually amazing how poorly ultra tropicals do in South Florida, even in warmer months. I have a few potted mangosteen seedlings that are three years old that are and inch tall. Mango seedlings will grow to five feet in one year without an issue. Durian will grow in a pot rather slowly and sickly, put it in the ground and it will die of phytopthera in weeks. Most garcinia will do poorly, rambutan and pulasan the same. Its not just about cold tolerance, its the climate in general that separates the growing zones (temps, soils, pH, disease, pests, and others).