Giving that client is the king (even when the choices are questionable), we will oblige.
Being in HDR does not always mean 600+ nits. In fact, content is usually 50 - 300 nits.
And why not? If the only display you have was a 1000 nits device, will you artificially limiting yourself to 50 nits?
Is as saying that you’re watching an IpadPro 5th generation constantly with the luminance down at 20% just because. If You look SDR on that device it is pretty up to 500/600 nits and it is not looking stupid, that is why the photographic ratio and intents are retained, independently of the absolute nit level.
However, as always, this is just my opinion, clients I usually work with, like this approach and therefore I oblige…..