Well, I can only tell you what happens when you manually change the resolution from 60 Hz to "ProMotion" in macOS.

Presumably ProMotion is a specific mode that the display/display controller/... first needs to be set to. If you start out in anything other than that and then switch to it later in the boot process, you'd get that blank screen for a second, and Apple avoids that by writing the startup mode to non-volatile memory.

Otherwise they could also just always start out in ProMotion mode – but then you'd get flicker every time you (re)boot into any other mode, like 60 Hz for people who prefer that.

If I understand correctly, ProMotion is the feature that allows dynamically changing the refresh rate without screen blanking. So the reason refresh rate matters in early bootup is that if ProMotion is not enabled, you get brief screen blanking. Setting the screen to 1 Hz in early bootup and then changing it to the user setting later would require screen blanking.

That's not a modeset though, that's just the display working as intended. Basically a VSYNC that runs at a variable clock rather than a fixed refresh.