I imagine what those custom brightness apps do is not magically increase the brightness, but change the various pixels' brightness in accordance to some method/algorithm such that you see what appears to be brighter whites when placed next to certain other colors.

It's not what is implied by the parent post - where the mac is limiting the brightness only to have the app unlock it.

...I'd have to say that seems like a radical reading of the text.

No; you can adjust screen brightness just fine with the built-in settings, including with the F1 and F2 keys (plus the Fn key if you've got them set that way).

This Vivid app is specifically for extra HDR levels of brightness. I've never had a problem with my M1 or M4 MBPs, either inside or outside, with the built-in brightness levels. (But, to be fair, I don't use it outside a lot.)