Is this why Arabian horses from Brazil did so well in the show ring twenty years ago?

The Southern Hemisphere birth date advantage that explains the Brazilian reining horse development phenomenon is exactly the same structural factor that contributed to Brazilian Arabian horses' competitive success in the international halter and performance show ring during the period you are describing, and the connection between the two observations reflects the same underlying demographic reality applied to a different discipline and a different era of international competition. Arabian horse showing at the international level — particularly in halter classes where horses are judged on conformation, presence, and physical development — is acutely sensitive to the age and developmental stage of the horse at the time of judging. A halter class judges a horse's physical form at a specific moment in time, and physical form at any age is a function of both genetic potential and developmental stage. Two horses with identical genetic potential for conformation and physical presence will look meaningfully different at the same calendar age if one is six months further along in his physical development than the other, and in the halter ring that developmental difference translates directly into the muscling, the definition, the body condition, and the overall physical presence that distinguish a good placing from a winning placing. Brazilian Arabians foaled in the Southern Hemisphere's spring — arriving at major international shows at six months of developmental advantage over Northern Hemisphere horses of the same calendar age — presented physically as more developed, more defined, and more mature than their Northern Hemisphere counterparts in ways that the halter ring rewarded with consistently high placings. The horses were not necessarily genetically superior to the American or European competition, though Brazil had also invested seriously in Arabian breeding programs with quality genetics. They were developmentally more mature at the moment of competition, and that developmental maturity expressed itself in exactly the physical qualities that halter judges evaluate. The broader Arabian industry's awareness of this phenomenon developed over time as the pattern of Brazilian success became established enough to prompt analysis of its causes, and the recognition that the birth date advantage was a significant structural factor — rather than a reflection of definitively superior Brazilian breeding — led to discussions within Arabian breed organizations about whether competition categories should account for hemisphere of birth.