As far as Tandem OLED, earlier OLED panels prior to the ones used by the iPad Pro actually stack OLED panels as well but not as elaborate as the ones branded by LG Display / Samsung today after certain breakthroughs of execution.The introduction of mini-LED launched displays like the TCL 8-series which had tens of thousands of LEDs in about 1000 zones, and this thousand(s) of dimming zones became one of the defining features of mini-LED.
In contrast, high end FALD TVs of the same time frame were in the hundreds of dimming zones. The Pro Display XDR, which came out around the same time frame as the first mainstream retail mini-LED TVs, had far fewer dimming zones than those mini-LED TVs, again measuring in the hundreds like other FALDs. To be fair, the Pro Display XDR was never marketed as a mini-LED display, and press did not refer to the Pro Display XDR as a mini-LED display either.
So you may think this is an arbitrary distinction and you would be right about that, but again based on numbers of dimming zones, all mini-LED is FALD but not all FALD is mini-LED. The Pro Display XDR falls into FALD category but misses out on the mini-LED subcategory.
I have no idea where you're going with this. Tandem OLED means 2 or more OLEDs stacked. In Apple products this only exists in the iPad Pro. Better tandem OLED in the future will also be tandem OLED, but better.
OTOH, yes the Pro Display XDR has more dimming zones than early FALD displays, but does not have as many dimming zones as the early mini-LED displays. Thus, it has never been categorized in the mini-LED category.
Same thing you’re suggesting with particular full array local dimming (FALD) displays being distinctly dense than others to more appropriately be branded the distinction of being Mini-LED FALD but not the ones less dense (Pro Display XDR including the very same trade-offs having one and other FALD more agreeably to you branded as MiniLED such as Asus Pro Art monitors) due to a particular amount of zones possible by modern breakthroughs of execution of the process.