It’s a bit more complicated than that. Apple’s implementation of HDR capabilities is called EDR and it works on all of their new-ish devices and displays, including the dimmer ones like your 600nit iPad. The way EDR works is that it always tries to use any available display brightness above the currently set level to display the whiter-than-white highlights. If there’s no such headroom (brightness set to 100 %), no problem, 600 nits is white and that’s it. If you set the brightness lower, e.g. in a dimly lit room, “white” can be only represented by say 150 nits of brightness, but if you play a HDR video and there’s a whiter-than-white highlight, the display can still use the 600nit max brightness headroom and make the highlight four times brighter than what your eyes and brain already see as white.