People are obsessing over the OLED black levels too much. For one: the XDR has MUCH higher typical brightness and MUCH higher peak brightness. The typical brightness will be 400-500 nits more than it would've been on an OLED iPad and the peak brightness probably about double of what an OLED would've been. What does that mean in practice? HDR movies and tv shows will look much more poppy and impressive on this display than they ever would've on an OLED. Blooming is an acceptable downside , MiniLED is still preferable over OLED for an iPad. Yes, for the living room an OLED might be better where you will watch movies at night with a dimmed room. However an iPad also needs to be able to provide a stunning HDR picture during the daytime, maybe even outside on the beach - or anywhere really. OLED would never be able to do that currently.
Secondly: OLED might not technically have blooming, but you WILL experience blooming even on OLED tv's in real life. Why? It's an optical illusion, but basically your eyes/brain will imagine blooming around bright white objects. If you have an OLED at home just try it; set a completely black background with only a white apple logo in the middle. The apple logo will appear to have a white glow around it; even though it doesn't if you block off part of the picture or pixel peep.
Bloom is gonna happen people; even on OLED; you can blame your eyes for that one.
HDR isn't about brightness.
HDR is about the combined effect of brightness PLUS darkness (creating contrast) -- or, the difference between the minimum brightness and the maximum darkness. I mean, it's even described in the name... high DYNAMIC RANGE.
OLEDs brightness on black is 0 - nil - making the contrast infinite. For comparisons sake, they often attribute the B specs maximum value of 0.0005 nits of black to it. With a minimum of 540 nits on the brightest end, you end up with contrast that's 20x higher than the best LCD displays at 1,080,000:1... and OLEDs are much brighter than 540 now... and again to reiterate, the brightness of black on an OLED being literally 0 makes the contrast infinite, it can't be quantified in the LCD contrast ratio sense.
In other words, your statement about "HDR movies and tv shows will look much more poppy and impressive than they ever would on an OLED" isn't just false, it's impossible. This screen doesn't even qualify for the high contrast spec for HDR, it just happens to be the best of the high nit spec.
It is absolutely better in an iPad than an OLED would be, but for image retention reasons. You can get in excess of 1000 nits peak brightness in an OLED, today - iPhone 12 happens to be one of those peaking at 1200. That isn't the challenge. If not image retention, then there must be too much waste on a single sheet of glass when cut down to however many 13" screens they'd need.