Is it though? HDR is mainly about colour space and dynamic range after all, not absolute brightness, and lowering the black level is the easiest way of increasing overall dynamic range (halving the black point is a much smaller change than doubling peak white, after all). I'd imagine even HDR400 True Black can exceed the HDR-minimum 13 stops of DR easily. At 1 nit of minimum brightness you'll need 1000 nits peak just to reach 10 stops (and 2000 for 11 stops, etc.), but cutting that to even 0,02 nits - 40 times brighter than the HDR400 true black permissible black level - sees you hitting 13 stops at just ~160 nits, and 15 stops at ~650 nits. Of course you'll need a reasonably dark room to experience this, and it certainly isn't suited for a brightly lit room. But whether something is true HDR or not is quite separate from whether it's able to display
perceptible HDR in any given setting. So if this has decent near-black performance and can hit 1000 nits for small flashes of light, it's probably going to deliver a pretty great HDR experience as long as the room isn't too bright. All the while, most high-end FALD LCDs can at best reach black levels of 0.05 nits, meaning they need much higher peak brightness to display the same dynamic range - ~410 nits to hit 13 stops, and 1600 nits for 15. OLEDs still really struggle in brightly lit rooms - that's why we went Samsung QLED in our living room - but you can't deny their HDR prowess.
Regular HDR400 is pretty much nonsense, especially as those monitors rarely have anything like FALD and thus don't come even remotely close to 0,05 nits black on a screen with a bright spot, but HDR400 True Black can deliver pretty good HDR as long as the room isn't too bright.
(For anyone wondering: contrast ratio is a pretty decent indicator of HDR performance for non-FALD displays, as it maps directly to dynamic range. 1000:1 contrast ratio = ~10 stops of dynamic range, 3000:1 = ~11.5 stops, etc. For true HDR you need at least 8000:1 effective contrast ratio, though that's highly dependent on how close your two measurement points are and the dimming zones of the monitor. Techspot/Hardware Unboxed's bar of 50 000:1 best case single frame contrast and 5 000:1 checkerboard contrast is a pretty good bar for good HDR - and AFAIK the only monitors to hit that bar are the Samsung Odyssey Neo G9 and that insane $4000 Asus one, with the Predator X35 coming very close, while an LG C1 48" hits it easily.)
This chart illustrates well how LCDs struggle to achieve good HDR results without cranking the brightness, while OLEDs can do it without breaking a sweat: